Is it ok to remove an upvote to an answer if you later see a better answer, and now you just think the other answer does not measure up with the new answer you consider as the better one?
If you think the new answer is so much better that it should float to the top in the "votes" sorting order, then I'd say you have no other choice than to remove the upvote to the other correct answer. (Downvoting that other answer is clearly not an option.) So I say: Yes, it's OK if you remove that upvote.
(You can of course just use Juan's reasoning and vote however you like!)