Good question - I'm not sure. I don't sit in the family stand for a number of reasons, too many kids :-), and upgrading tickets is a pain, the other is when your kids are too old for the family stand if there are no available season tickets then you go onto the waiting list.
I suppose that...
If the tickets are in the family stand you must have a child with you. If you upgrade the child ticket to adult then if there is availability elsewhere in the ground both seats will be relocated. If there is a cost difference between the adult seat in the family stand and the relocation seat...