I have allowed Players to change their skills and powers as they wish between games. The idea is that as the character is roving around the world they may choose to concentrate on some skills they know over others. This is difficult to rationalize from a story perspective, but not impossible. One might be concentrating on some skills over others by practicing what they learned. However, what I found is that too much swapping of skills and powers does cause story problems in terms of continuity. So there's the revised rule:
Rule: Players can change one skill or power between game sessions.