Fully echo the comments about letting GAs or FAs know about it. We have seat swapped a few times but always let the FAs know and usually on emptier flights. Just came back from AMS-JFK in C+ but we had aisle/window and then moved to the wide open middle row after letting FAs know
I do recall a DTW-PEK flight where I gave up my D1 seat to give to my traveling companion and let the FA on the upper deck know and I took her C+ seat. For some strange reason, the FA did ask for us to swap back before landing though.