They will cancel the upgrade, do a date change in J to a saver J on the new date (with a fee) then upgrade again to F
I'm not sure about that, BUT you need Z (F Upgrade) to be available on the new date, and they'll charge the J Saver change fee. Guess it depends a bit on how the agent does it, there seems to be an automatic system (which might cancel and refund) and a manual way, where the agent can just amend the date in the existing segment of the PNR