Campden tablets don't kill yeast, so I assume that the apple concentrate will keep fermenting until the corks blow.
You could uncork and then either very gently pour back into a fermenter and let it finish up, or you could uncork and then recork.
Campden tablets are a great antioxidant and preservative, so you may have good luck with putting the cider back into a carboy without oxidizing it.
If you want to sweeten a wine or cider, you can use potassium sorbate when fermentation is finished. You usually add the campden along with 1/2 tsp sorbate per gallon, and let that sit for a few days. (Rack the cider into the carboy into which the campden and sorbate have been dissolved with a little water). After a few days, you can sweeten as desired. Wait about three days to ensure fermentation doesn't restart, and then bottle.
Sorbate doesn't kill yeast, either, but it does inhibit yeast reproduction. So you may have a little sign of refermentation while the few yeast that are in here try to ferment your added sugar, but since they can't reproduce because of the sorbate, it should stop very quickly. That's why you wait a few days before bottling.
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