Originally Posted by IAH-OIL-TRASH

It's just a 1:1 code swap - should be no fare change.
NH actually processes the upgrades and issues upgrade success notifications, not UA - so maybe not instantaneous.
If upgrade not forthcoming prior to 24 hours out, no upgrade.
Not sure about NRT F lounge access based on arrival F, departure J (I am just aisle traffic in ANA F)