There are various facilities within the game that allow you to swap powers out and such. Mostly these are fine as is BUT, it is very important that each and every time you swap these powers you inform the GM. Novel play is the only way to surprise the GM. If you have some item or power swapped without telling me it will be disallowed when you attempt to use it and the only one that gets surprised will be you.
There are possible ways to get the effect of retraining and these involve dangerous or epic events or massive magic. Do not count on such a thing to shift your power or feat later on. It's unlikely.