TMK, the smaller coins started to pop up about 3 months ago. The FamilyMart near my station entrance (there must be at least 5 of them inside Xujiahui Stn) happily lets me swap them for old coins. When doing this drill, I usually bring my y.5 coins along as well because lots of ticket machines seem to dislike them (well, if you have ~10 of them and only need 6 to get a ticket, you'll probably pull it off, even when using the most stubborn machines).

On a semi-related note, I have one of the subway payment apps, but I stopped using it 6 months ago when I realized the extent to which app users gum up the system (e.g. they spend time launching the apps and the QR codes often don't scan very well).