I'm in the middle of reassembling my rear subframe for my M2 project and am wondering if theres a reason to change out the stock wheel studs. Nows the time, if so.

I've done the 320i rear drum upgrade soooo

Any thoughts ???

Unless they are otherwise damaged, they should be fine. I do, however, replace the nuts at intervals depending on useage. The race car gets new ones annually and the street/track cars get replacements every 12 events or when retorquing after lap sessions becomes significant.
I buy open nuts to fit Chrysler products, M12-1.5x19mm, open faced for about 70 cents each (Dorman Products part # 611-063).

For trips to the grocery, oem nuts will probably last forever...........