There's a pretty good case for either of them IMO. I'd enjoy reclassing Olivia and Naga's Tear will help give her that extra boost especially since she got extremely Strength screwed (5 Str as a level 15 Dancer... yuck). However, even with that strength screwage, she should level fairly quickly once I reclass her as a Myrmidon, and with improved strength growth, I'm not too worried.
For Frederick, I just feel like he needs the stat boosts to make up for how his stats are so crappy relative to his internal level (unless I'm misunderstanding how internal level works). Basically I plan to reclass Frederick when he hits level 16 GK, and at level 13 GK, his stats are pretty much equal to the weaker level 1 promoted units I have.