Originally Posted by
Lux Flyer
There is the ability to temporarily allow overbooking to get additional segments sold, whether that be to correct a previous agent error, or to help break married segment logic. However it is highly unlikely that anyone would authorize overbooking, even momentarily, for 1 J seat on an international flight, let alone 4. For the same reason as above, the instant they release that restriction on overbooking, those additional spots become fair game in the system.
A UA reservations agent should be able to override the lack of inventory and force new segments regardless of inventory without opening J space to anyone else. This does require special authorization to sell beyond J space, but should be standard procedure in the case of any INVOL rebooking where the human can see the pax already has the segment HK in that cabin. (requires a decent agent, hence sometimes HUCA)
But it sounds like OP is making a voluntary change, so this is moot - I agree that it's extraordinarily unlikely someone would do this for a voluntary change.