Tzitzis – Bigdei Shabbos
If one purchased a new pair of tzitzis, and his old one is still kasher. Should he wear the new one on Shabbos to honor the Shabbos, or during the week to fulfill mitzvas tzitzis b’hiddur for the whole week? And if he should wear the new ones during the week, would it be proper to change into the old ones for Shabbos, or he should keep on wearing the new ones?
Being that they are both kosher tzitzis and the issue is wearing a nicer garment, the nicer one should be saved for Shabbos. They may be worn for during the week and Shabbos as well. If they are being worn during the week, they should be kept on for Shabbos as well. There is a midash chassidus to have a separate [and nicer] Talis katan for Shabbos.
see Kitzur Hashlah Shabbos pg 152, Maaseh Rav 147, Nishmas Shabbos Vol. 1 Siman 277.