If someone has a number of tallit koton, with tzitzit some of which are no longer kosher, is there anything which states that one is required to replace the strings with kosher ones if the garment no longer has the four corners? I understand that wearing tzitzit on a four cornered garment is a mitzvot; I ask because I wonder whether it is permissible to remove the broken/unkosher tzitzit and to sew the garment together so as to remove the four corners? I wonder about the obvious possible uses of the garment itself if it did not have the four corners in the same way we "reuse" those objects like the lulav and etrog which, after Succoth is over, they can be reused toward another miztvah because they are not longer considered (by some) to be holy.
Let me rephrase my question: I understand that the tallilt katan is a piece of garment specifically made and designed to fulfill the mitzvah of wearing tzitzit. However, if the tzitzit are not 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?