Gemara

(Beraisa): If one brought a second year ram for an Asham that must be a yearling, or vice-versa, it is valid, but the owner was not Yotzei;

R. Shimon says, it is not Kodesh at all.

Question: R. Shimon says that Mechusar Zeman (an animal that cannot be offered now) gets Kedushas ha'Guf!

Answer #1: There is different, for it can be offered later.

Objection: If so, also a yearling Hukdash for a second-year Asham should get Kedushas ha'Guf!

Answer #2: Rather, R. Shimon learns from Bechor:

(R. Shimon ben Yehudah citing R. Shimon): An animal less than eight days old enters the pen for Ma'aser even though it is Mechusar Zeman;

We learn from Bechor. It becomes Kodesh (at birth) before it can be offered, and it is offered in the proper time (after eight days).

Sukah 4a: If a Sukah was above 20 Amos tall, and one built a platform in the middle, it is Kosher only if it is within four Amos of each of the walls.

9a (Mishnah - Beis Shamai): An old Sukah is Pasul;

Beis Hillel are Machshir.

An old Sukah is one made 30 days before Sukos. However, if he made it l'Shem Sukos, even if he made it at the beginning of the year, it is Kosher.

11a (Mishnah): If one draped a vine over the Sukah (for Schach), it is Pasul. If he cut it, it is Kosher.

(Rav Yosef): Rav and Shmuel say that also, one must move (lift and put down) the Schach after cutting it.

(Rav Huna): Rav holds that one need not move it after cutting it;

Rav Amram Chasida inserted a long folded string through a garment, without cutting it (to make four strings with eight ends). R. Chiya bar Ashi told him that Rav holds that it suffices to cut the string now.

This shows that cutting is like doing. The same applies to Schach!

Rishonim

Or Zaru'a (2:289:2, cited in Hagahos Ashri Sukah 1:24): R. Baruch says that if one made a Sukah in the house under the roof, and afterwards removed the roof, it does not help, since initially it was made Pasul. The Torah requires Ta'aseh v'Lo Min ha'Asuy. R. Yitzchak ha'Lavan was Machshir. Ta'aseh v'Lo Min ha'Asuy applies only when the Schach itself was Pasul, e.g. an attached vine. Here the Schach itself was Kosher, just the roof disqualifies. This is correct. Even the vine becomes Kosher if one cut it and moved it. All the more so this applies here, for the Schach itself is Kosher once the roof is removed.

Me'iri (Sukah 3b): Since the Sukah was Pasul when the Schach was put on, how does it help later to diminish the height (through a platform)? This is not Ta'aseh v'Lo Min ha'Asuy, since the Schach was put properly l'Shem Sukah, and later an action fixed a different Pesul. If a Sukah was Pasul due to four Tefachim of Pasul Schach, and he diminished it, it is Kosher, even though he did not do anything to the (already) Kosher Schach. According to the opinion that a Lulav must be tied (to the Hadasim and Aravos), if one tied Hadasim with too many berries and later diminished the berries it is Pasul, for it was not tied properly. Gedolei ha'Doros (usually the Me'iri uses this title to allude to the Hashlamah, but here the Hashlamah holds like the Me'iri) wrote oppositely; I disagree. If one made a Sukah under a roof and removed the roof, one need not move the Schach. One must move it only when the Schach itself was Pasul, e.g. an attached vine, if we do not consider cutting it to be the action of making it Schach.

Tosfos (19b DH Af): In Bechoros (21b) we suggested that we should learn from Kodshim, in which Mechusar Zeman does not become Kodesh! We say (e.g. Zevachim 12a) that one may be Mekadesh a Korban on the night before the eighth day, and offer it the next day. This connotes one cannot be Makdish it before this! We can say that this is l'Chatchilah, but b'Di'eved if one was Mekadesh it even earlier it is Kodesh, for we learn from Bechor.

Poskim

Rema (OC 626:2): One may make a Sukah under Mechubar (an attached tree or vine) or a house, and afterwards remove (the Mechubar or house), since the Pesul is not in the Schach itself.

Gra: The Rashba (really, Ritva) (Sukah 10a DH v'Heicha) explains that Rav Ashi permitted leaving a wet garment on the Schach in order to dry when one is not eating or sleeping in the Sukah, and remove it before eating or sleeping there. This shows that removing it helps. Even though the Rashba said that some disagree and are Machshir anything not needed for the Sukah, and the Yerushalmi supports them, he agreed with the first explanation, and the Bavli supports it. Also, perhaps the proof applies even according to the latter Perush.

Mishnah Berurah (15): Even those who argue with the Rema below (Sa'if 3) agree here, like the Bach and Magen Avraham say.

Rema (3): One may make a Sukah under a roof that can be opened, and close it at the time of rain and open it afterwards. This is not Ta'aseh v'Lo Min ha'Asuy.

Beis Yosef (DH Matzasi) Maharam bar Baruch (182) says that people say that one may not make a Sukah under a roof before removing the roof, due to Ta'aseh v'Lo Min ha'Asuy. R. Elchanan says that Ta'aseh v'Lo Min ha'Asuy applies only to an old Sukah, but not here. If a Sukah was too high, we diminish it. Similar, here one need not dissemble the Sukah and Schach before removing the roof.

Beis Yosef (DH v'Zeh citing Orchos Chayim Sukah 26): People who make a Sukah in the house and remove the shingles (roof tiles) only afterwards, they need not move the Schach due to Ta'aseh v'Lo Min ha'Asuy. Removing the tiles and Pikpuk ha'Dapim (moving the boards) is an action to be Machshir the Schach that was made in a Pasul way.

Bach (DH Od): Maharil says that a Gadol said that if the entire year a Sukah has a slanted roof, and during Sukos they remove it to the side and put Schach there, one must remove the roof before putting the Schach. If not, it is Ta'aseh v'Lo Min ha'Asuy. Maharil disagreed, for Or Zaru'a permits putting Schach under the roof. I say that we can distinguish. Or Zaru'a discussed a proper Sukah, just it is under a roof. The Schach is made b'Hechsher. In the Gadol's case, it is not a Sukah until the roof is removed. Therefore, one should be stringent. I received from Maharshal to be stringent for Ta'aseh v'Lo Min ha'Asuy if the roof was not removed beforehand.

Magen Avraham (7): I say that one who is stringent like Maharil is Machshir when one removes the roof later, for this is an action, and it is Machshir the roof itself. Here, opening (the roof) is not considered an action. However, if he first put the Schach b'Kashrus, then closed the roof due to rain, and then opened it again, all agree that this is Kosher. This is like spreading a sheet and removing it. It is Kosher, since the Sukah was made b'Kashrus. Therefore, one should be stringent (not to put the Schach while the roof is closed). Shiltei ha'Giborim (Sukah 8a:1) is stringent.

Mishnah Berurah (18): The Bach, Magen Avraham and Acharonim say that one must open the roof before putting the Shach. If one put it when it was closed, one must open it and move the Schach, i.e. pick up each piece and put it down for the sake of shade. If one put Schach on the roof, and then removed the roof, all agree that it is Pasul, for it was not called a Sukah when on the roof, since there were not walls (Pri Megadim - it was not a Sukah at all). Therefore, removing the roof is not Machshir.

Kaf ha'Chayim (36): If one put Schach when the roof was closed, and did not pick up the Schach and put it down before Yom Tov, one may rely on the lenient opinion to eat and bless, for it (the stringent opinion) is a mere stringency.

Pnei Yehoshua (Sukah 3b DH Haysah): Why does it help to fill in a Sukah above 20 Amos to diminish the height? This is Ta'aseh v'Lo Min ha'Asuy! This is good for R. Tam, who says that a Sukah that is too tall is not a Pesul in the Schach. However, Rashi considers it to be Schach Pasul. However, I answer that the Pesul is not due to the Schach itself, rather, to the air underneath it. If so, in any case it is Asiyah. Diminishing the airspace suffices. Since the Pesul was not due to the Schach itself, one need not do an action to the Schach itself. However, Maharam bar Baruch (182) connotes unlike this. This requires investigation.

Gilyonei ha'Shas (Sukah 3b DH u'Va): The Maharal (Gur Aryeh Eruvin 2a) says that Ta'aseh v'Lo Min ha'Asuy applies only if the Schach is not considered Schach at all, but not if it is Schach , just there is a Pesul.

Yabi'a Omer (8 OC 3): It seems that Ta'aseh v'Lo Min ha'Asuy disqualifies only when the object is intrinsically Pasul, e.g. a three cornered garment. It does not apply when the object later becomes Kosher automatically. Ta'aseh v'Lo Min ha'Asuy applies to a Sukah, e.g. if one covered a Sukah with an attached vine and cut it, and normally a Sukah is made before Sukah (before the Mitzvah applies). If it was made l'Shem Sukah, it is Kosher even from the beginning of the year. Even though we learn from "Chag ha'Sukos Ta'aseh Lecha Shivas Yamim" that one must make a Sukah proper to last seven days (i.e. one must make it before Sukos), we learn from there that Ta'aseh v'Lo Min ha'Asuy disqualifies only what is intrinsically Pasul, but not what depends on time. Even R. Baruch (in Or Zaru'a) disqualified only because he removed the roof after putting the Schach. An action needed to be done to be Machshir. Mechusar Zeman is different, for no action needs to be done. All are Machshir. Hagahos Ashri and the Rema (626:3) rules like Ri ha'Lavan. Sefer ha'Michtam and the Me'iri say that an action is needed only if the Schach itself was Pasul. Maharam bar Baruch (182) says that R. Elchanan is Machshir a Sukah made under a roof, after the roof is removed. He proves this from the platform used to be Machshir a Sukah.