It can be done by Revenue Management, ie creating 4 U class seats that don't exit, and then cancelling the 4 you have in a separate booking to compensate.

It can be done, but it requires a special request. This may depend on your status.

I would call again, and ask the agent if she can send a special request to Rev Man to do this. But possibly the answer would be no.

If you have to cancel and rebook. Don't have the same agent do both as there will be a delay of several minutes. I would ask them to get a colleague to cancel as he/she grans the seats a second later.