Question: After replacing the tallit I have used for Shabbat, may I use the old one for during the week?
Answer: We find in several contexts a concept of ma’alin bakodesh v’ein moridin (=ein moridin) – one may only raise the status of a sacred object (or person) but not diminish it. The question is whether this case falls into that halacha.
The Beit Yosef (Yoreh Deah 259) cites the Mordechai who says that an object that was used in a mitzva role, e.g., a candelabrum in shul, can be switched to another mitzva use, even if the latter is of a lower level. He explains that ein moridin applies to tashmishei kedusha (objects that "serve holiness," especially sacred scrolls), but not to tashmishei mitzva (objects used for mitzvot that do not include such articles). Some Acharonim (including Maharsham II:39 and Yabia Omer II, Orach Chayim 1) posit that since tzitzit is a classic tashmish mitzva (Megilla 26b), ein moridin does not apply to it; one just must avoid disgracing it (Shulchan Aruch, OC 21). They are aware that important sources refer to ein moridin regarding tzitzit. This is either because they represent opinions that argue with the Mordechai (see Darchei Moshe YD 259:3; the Shach YD 259:11 supports the Mordechai), because tzitzit is more kadosh than a shul’s candelabrum, or the opinions represent acts of stringency (Maharsham ibid.).
The Shulchan Aruch (OC 15:1) rules that one may take kosher tzitzit off one garment as long as he will put it on another one. Most Acharonim (including Beit Yehuda (Assad) YD 29, Mishna Berura 15:1) permit moving the tzitzit from a tallit gadol (what we call a tallit) to a tallit katan (what we call tzitzit). Those who forbid it invoke the concept of ein moridin, saying that based on halachic or Kabbalistic factors, a tallit is of a higher level than tzitzit (see discussion in Yabia Omer ibid.; Tzitzit, p. 286). The majority opinion does not dismiss the relevance of ein moridin but denies there is a fundamental difference between the mitzva for the two garments. On the other hand, the Magen Avraham (8:6) and the Mishna Berura (8:9) explain the minhag of having an atara on the tallit as aiding one to keep the two front tzitzit in their more prominent position. While they cite the Ari’s practice to not be careful to maintain the positions, the stringent opinion is quite accepted. Since we see that it is not always obvious to determine when a change is a prohibited diminishment, it is worthwhile to seek additional grounds for leniency.
The Taz (OC 154:7) suggests that if one has decided to stop using a holy object for its designated purpose, making it a candidate for geniza, it is better to "lower" its usage than to totally take it out of use. Thus, since it is appropriate to periodically upgrade a Shabbat tallit, it would be better to use it at least during the week. However, the Taz is not widely accepted (see Sha’ar Hatziyun 154:23). Another possible strategy is to initially stipulate the intention to eventually transfer it to weekday use. However, this idea is apparently contradicted by the fact that the gemara (Yoma 12b) does not raise it in regard to the need to retire rather than "lower" a temporary kohen gadol (see S’dei Chemed vol. V, p. 109).
Despite scant authoritative discussion of the topic, we would confidently permit one to transfer a tallit from Shabbat use to weekday use for the following reasons. It is unclear that a Shabbat tallit has more mitzva importance than a weekday one; consider that the frequency of use is halachically prominent. More fundamentally, even if a Shabbat tallit is more prominent, it is not in regard to the mitzva of tzitzit but because wearing nicer garments is a means of honoring Shabbat (Rambam 30:3). Therefore, buying a new tallit and using the old for weekdays is like buying a new suit for Shabbat and using the old one for weekdays - a practice we have never heard questioned due to ein moridin.
Thus, based on several reasons, one should have no compunction about using a former Shabbat tallit for weekdays.
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