We're constantly changing. I believe the thing you're asking though, is whether we can consciencely force ourself to change into something we want to become. This poses two interesting questions: who does the changing and who does the forcing?

I think it is possible. Of course, you can change how you act by watching yourself and adjusting your behaviour, but the motivation has to come from somewhere within yourself and you're not really changing who you are and what you want, just how you act. This is by no means easy or anything, all I'm saying is that I do not consider it actual changing of who you are into someone else, just better expressing who you are and want to be.

You can change, though. If you do something long enough, it will not only change the way how you act, but also feel and think. A common exercise for people with selfesteem-issues is making a list every day of positive actions that day: in time this should help them see there's good things about themselves which they're missing and change the way they think and feel about themselves. This is a good example of really forcing yourself to change the way you feel, think and thus who you are.

I believe it is possible to actively change who you are. But it's by no means something easily done. The motivation has to be really, really strong, otherwise you'll keep failing until you do have the will to change.

An interesting question, though.