I'm a series newb and I am a bit confused regarding the precise mechanics of reclassing. What confuses me in particular is reclassing a charachter and how it affects promotion later on.

Example, let's say we're looking at someone like Gregor in FE:A. He can switch in to the myrmadon and barbarian lines. Should I eventually want to promote him into a Hero or other second tier class in his original line, do I have to switch back to his original class in order to do so.

Also, any help in terms of how to approach reclassing/promotion in general would be great. I'm new to the series and just some tips in terms of what to consider when planning out promoting and reclassing units would be much appreciated.