As part of the bumping process, they rebook you.

If you're in F, they try to rebook you in F.

If they can't, they tell you then.

If you don't like it, you can say "no thanks."

On one occasion, I volunteered for bump and was rebooked from a trans-con non-stop to a short hop up the west coast followed by a non-stop. They were able to put me in F for the long (SFO-BOS) let but not for the short one, so they gave back the certificate I had used to upgrade.