Suppose a user achieves 3000 reputation points and begins casting close votes. Then, the user downvotes an answer and loses 1 reputation point, falling to 2999. What happens to the close votes? Do they get canceled or do they stay?
The votes still apply. As the user had the prerequisite amount of reputation when they cast the vote, there's no reason to discard it. In fact, doing so would be rather expensive. It's not at all uncommon for someone to reach a new milestone and lose it for a few hours or days until they get a few votes over the line.
This is not really unlike binding votes that a moderator cast holding even after the moderator has stepped down and no longer has the diamond privileged. If you look closely through almost any site, you'll see at least one question that was closed by what appears to be a single ordinary user that doesn't have the necessary reputation to cast a normal close vote.
Hopefully, no votes or reputation were donated to science prior to me answering :)