If the existence of a god or gods became clear, then we would need to update that definition.
By the definition of agnostic above, both theists and atheists can be (and likely are) agnostic. After all, we are describing beliefs, even though many religious people (and no small number of non-believers) are confident that they know, not just believe. I think our minds work this way, where we are uncomfortable not being sure. Although I do not write off the possibility of a god existing, I live my life as if I am sure none does.
If Yahweh were to announce his existence in the way that is typical for him, neither the atheist nor the agnostic would be given much time to make any adjustments in their thinking.