It may not be they are impostors. There are over 10,000 profiles on the board, so choices are more limited and it is bound to happen that one will be close to your's.
Anyway, you cannot change the username. The software needs something to be a constant to associate everything else in the profile to. So the only thing to do would be to delete the current and add a new, that may have it's risks too! You may not get what you want.
-John