The policy is you should be re-booked in F, but most agents are oblivious to this. I can only assume this arises from the fact that doing this requires special processing new agents don’t know how to do.

I quote this policy, which states you should be put in the same Class of Service (ie fare class) if available, and if not available you should be put in the next fare class in the same cabin (ie First class).

If they refuse to do it, kindly request they transfer you to Global Ticketing Services or International/Manual Reissue (IMR) where you will find a knowledgeable agent who can do this for you.