AbdulAlhazred
Legend
In the latest D&D Podcast they talked about this in regards to Paragon Paths. Someone had asked if it was ok to change Paragon Paths or Epic Destinies once they were chosen, since they couldn't find a written rule or mechanic about. They're answer was basically 'Why the heck not?'. They professed that they didn't see how it would effect balance. So as long as it made since in game, go for it.
The same could be used with themes. In fact, with some themes it just makes sense. If you get bit by a vampire at 11th level, the easiest way to represent the change is to drop your Gladiator theme and add the Vampire theme. It doesn't mean you were never a vampire, but this new condition has pushed that part of your personality down a bit.
Sure, but it isn't all that profitable IMHO to discuss what we can do OUTSIDE the structure of the rules of the game in a discussion of the design of the game . Not that I disagree with you. The questions are more about how can the rules be structured to provide that kind of flexibility OOTB? Obviously there are limits to what the system can lay out and creative hacking on it is always a powerful tool, so there need not be perfect answers.