Is there anything wrong with buying a new hat (or suit, shirt, pants, socks, shoes, skirt, dress) for Shabbos, and then taking one's old Shabbos hat (or suit, shirt, pants, socks, shoes, skirt, dress) and downgrading it for use on weekdays?
On one hand, this seems to be the Middah of Shammai Hazaken (Beitzah 16a):
תניא אמרו עליו על שמאי הזקן כל ימיו היה אוכל לכבוד שבת מצא בהמה נאה אומר זו לשבת מצא אחרת נאה הימנה מניח את השניה ואוכל את הראשונה
It is taught in a baraita: They said about Shammai the Elder that all his days he would eat in honor of Shabbat. How so? If he found a choice animal, he would say: This is for Shabbat. If he subsequently found another one choicer than it, he would set aside the second for Shabbat and eat the first. He would eat the first to leave the better-quality animal for Shabbat, which continually rendered his eating an act of honoring Shabbat. (Sefaria)
On the other hand there is a concept of :מעלין בקודש ואין מורידין and therfore it would be inappropriate to take something designated for Shabbos and downgrade it to weekday. (Perhaps the case of Shammai is different since the food was not yet actually used on Shabbos, just designated for Shabbos and הזמהה לאו מילתא although, if so, perhaps having Shabbos leftovers during the week would be a problem...)