Yes, for operational reasons passengers will be asked to move around but a pilot will tell FAs of such need. Least on pre-merger AA pilots and FAs were not allowed to upgrade passengers on their discretion. I posted once about an empty 738 flight I got upgrade and after take off FA on board told a pilot and FA sitting in main cabin that they can move up to first class. AA FA responded to my post and said at AA that is a big no no, pilots of FAs can get in serious trouble doing that.

EAAagle FA told me once that when EAAgle first introduced first class on CR7 many EAAgle FAs were not familiar with first class procedure and many did not read carefully of first class procedure it was given to them. This FA told me that she knew some FAs at EAAgle got fired because they upgraded a passenger to first class on their own discretion.