Again, the rules are written expecting PC adventurers who are earning XP through adventuring. They're not intended to simulate circumstances like three hundred years of intense study -- that's stuff for NPCs or perhaps backstory. So if you get into those circumstances as a DM, you have to decide what seems reasonable. Maybe you use the downtime training rules notwithstanding their official limits. Maybe you use _re_training rules and have the elf change a feat or even a level he already has, ruling that his existing skills have atrophied because of his focus on this new field. Or maybe you just say that three centuries of study does net you a sum of XP. It's up to you.