I have absolutely no idea how it happened, but my change took.
Edit: I figured out the sequence of events... I clicked "Submit," and the page refreshed and told me that my changes had been accepted (or whatever), but when I scrolled to the bottom of the refreshed page my old avatar was still there.
When I posted, my old avatar showed next to the post.
Some time afterward, I clicked on My Stuff>View Profile and saw that the change had taken hold.
Something to do with server response, or a UBB issue?