Using a queen excluder will not work.  The old queen will stop her egg
production and slim down enough to go through the excluder.

I would make a split.  If the hive if far enough in their planning to swarm,
there is nothing you can do, but hope to catch them.

My hive swarmes three days ago and passed through four yards before coming to
rest in a low branch.  My neighbors were not too thrilled but a couple of
jars of honey to each helped.

Norm