If choosing between A and B, is it possible to later move from A to B or vice versa, and if it is, which one is harder? If it's trivial to go from A to B but no

If choosing between A and B, is it possible to later move from A to B or vice versa, and if it is, which one is harder? If it's trivial to go from A to B but not the other way around, I'd have to have very compelling reasons to choose B. I use this just aboute very week in software development.