But there would be a vacancy if the fella had done what you suggested Doug, and resigned as senator with the intention of asking the voters to back him. Except that they wouldn't have a chance to back him, because the governor would appoint the replacemnt instead.
Every time a member switches parties in the House of Commons there are objections that it's unfair and they should resign and see if the voters would reelect them. That has happened, but very rarely. Winston Churchill was the most famous turncoat, did it twice.