I assume that expired close votes are "locked" similarly to the way this meta.SO post says that cancelled up or down votes are locked.this meta.SO post says that cancelled up or down votes are locked.
For up or down votes, 5 minutes after you cast them, they're locked and can't be changed unless the post is edited. According to that linked post, if you upvote something, then cancel your vote (by clicking the up arrow again) you're still considered to have voted and that's locked in after the 5 minutes.