Doesn't need to be a storyline justification for the learning of skills. Learning skills as you gain experience is a natural thing that logically makes sense to people. This even happens in real life. You do things, you learn things. However, "unlearning" things then retroactively learning different things is not something that can be justified at all in a "common sense" manner.
Nor does it happen in real life. I'd be very surprised if after learning how to use a sword well, you switched the battleaxe and all of the sudden forgot how to wield a sword effectively in real life.
