ime, it's a complete ymmv situation based on variables such as flight loads, irrops and such. for example....

i was booked on a 6pm departure sfo-san in f on 57 and the a/c was swapped to a ps 57. olci'd at t-24 and i was watching the flight for a possible bump* prior to heading to sfo and noticed the 57-ps57 swap (and confirmed by unitedcargo) at around 2pm. i logged into olci and and saw that my seat was changed from 5b (57-f) to 6b (ps-c) (which i promptly changed it to 9c ) so it can be done but again, ymmv.

*didn't get the bump but was opped to f but i digress....