1) A true full-J fare can instantly confirm into any flight for which there is J inventory > 0. Standby is only relevant if J = 0.

2) The gate agent at RSW the other day wouldn't let full-Y Latitude fares confirmed on a later flight get onto an earlier not-full flight with empty seats 30 minutes out, even with no bags. So 'standby' might not be of as much value as one thinks if the employees are intransigent or not respectful of the rules when a flight is under local airport control.

3) Eupgraders prior to flight time are confirmed into "R", or are put on a standby list for the same. It is possible, but uncommon, that filling "R" inventory as it is made available will ever truly deplete the actual J cabin. By confirming into J, you'll be ahead of everyone who isn't confirmed. Unless deadheading captain or AC execs show up, or the J cabin is oversold, in which case, even a confirmed J reservation can be downgraded/DB'ed (although they're supposed to downgrade the confirmed upgraders first!).