Although the interface looks like you can change your vote, it doesn't seem to actually do it. That's something that you might like to feed back to Discourse. (ProBoard's poll facility seems to work better.)
I see, that is a bit annoying, would need to tell users that they can't change their mind.
I thought it was doing something a little odder than that in that after you voted it displayed the results before counting your vote. So you can change your vote but it only shows up when you re-examine them not when you vote. I could be wrong though but I'm sure I was finding some odd behaviour when playing with them yesterday.
Yes, that does seem to work. Change your vote then refresh the page and you see the changed result.
Still sounds like slightly poor design though. Showing you what it was before you voted after you voted doesn't seem to serve much purpose.
I thought I'd tried that, but I've just tried again, and it did eventually show the changed vote.