I understand that the tallit katan is a piece of garment specifically made and designed to fulfill the mitzvah of wearing tzitzit. However, if the tzitzit are no longer valid and can be removed, and the tallit katan is still wearable, is there an obligation to replace the tzitzit, or can the sides of tallit katan be sew up to make it into a regular (non-four-cornered) piece of clothing and/or used for other mitzvot?
Thank you for your question.
Although a tallis katan, (called tzitzis) is usually made specifically with four corners, etc. in order for us to capitalize on the opportunity to perform the mitzva of wearing tzitzis properly, however the garment itself is not inherently holy. After the tzitzis are worn out and are not longer kosher, they ay be removed, and the garment can be cut in a way that it no longer require tzitzis when worn. The only thing is that the garment that was once used for tzitzis may not be used for a degrading use, such as to use as underwear or as a rag to wipe up dirty thing, etc.
O:CH 21-2, M:B 21- 9,11,12.