You're referring to "rolling" the cabins forward, a process that happens when Y is oversold and they need the seats. So they roll forward elites and full-fare pax who are ticketed in Y, putting them in J. If J is now oversold, they roll forward elites and full-fare pax who are ticketed in J, putting them in F.

If you're lucky enough to have it happen to you, make sure the GA confirms that the upgrade is complimentary and that s/he is not pulling any more miles, eVIPs, or stickers to upgrade you. Get verbal confirmation, and preferably write down the GA's name, in case you have to file a claim later.