Interesting one, but given the context of the situation I really doubt it is the start of "something new" in terms of overall policy.

You were told that J was overbooked, so clearly there were always going to be some op-ups. And as a Plat there was also every possibility you would be firmly in their sights - even if you had not discussed the issue at check-in.

But the exchange that then took place later at the gate is, I feel, absolutely no indication that merely asking would necessarily bring similar results in the future. It just so happened that your request (and that of the pax in front of you) suited them on the day, because they needed to move a number of pax anyway and it gave them the chance to say ..... "look how good we are to you, you only have to ask and we give you what you want" .......

Smart tactics on their part, but sort of disingenuous at the same time. IMHO.