yes. just did this for the first time a few weeks back (upgraded from A to J with PP) on A-fare tickets purchased via the Chase portal. no issues.

on the other hand, i'm currently trying to get 261 credit and downgrade credit (both from UA) for the canceled return flight. no PP seats were available on the rebook so i was booked into Y (did successfully upgrade to J, but of course using 40 PP instead of 30). very curious to see how these credits go. totally willing to take ETC.