But you're talking from a position where it is currently not possible to prove the existence/non-existence of a God. If, for example, God were proven to exist

But you're talking from a position where it is currently not possible to prove the existence/non-existence of a God. If, for example, God were proven to exist then the definition of an Atheist would not change. However the people who were Atheists would absolutely change - myself for one :) because many current Atheists are also rationalists.