It has been mentioned many times on this forum that a YF is technically NOT an upgrade and this has been put in writing by the customer service group at Hertz. It is not an upgrade from F that is, however it would be an upgrade from Midsize. I have had this mentioned to me on at least 2 occasions and pushed back to the agent. While they have been told this is an upgrade by the site management a quick call to Hertz CS certainly does the trick.

In the one case where I was unable to convince the desk agent after hours I had them downgrade the reservation to Midsize on the spot and at least saved the $$$ on the reservation at the time. When I returned the car I talked to the manager and they apologized for the confusion while promising to make the policy clear to the staff for future reference.

Cheers!