Thank you; however, there's no refund coming my way! Fortunately, it looks like the 'amend' button appeared as soon as my State return was accepted (the federal return still has not finished processing, since my account has not yet been debited). It was a bit frustrating, though, because the amend button did not actually allow me too amend the return. It just popped up a notification saying that the tax form to amend returns will not be available until 26 February!
So, for future reference, it seems like the best answer to my original question is: "The amend button will appear at some point after the return has been accepted, while it is in process; however, it won't matter for early filers, since you can't file an amendment until the form is finalized anyway, normally in mid-to-late February."
