Here's what I would do, son.

First, tell her you switched the flights so you both are flying together on the same plane and she doesn't bug you about it.

Next, monitor the inventory between now and December (you have LOTS of time).

If something opens up to your liking, grab it; if not, play dumb on the date of travel..."oh geez honey, I knew this was gonna happen, the airline screwed up..."

At that point, it's too late to change anything and she'll just have to deal.

Frankly, unless Eva Longoria married me...I would NOT give up a biz seat on another aircraft for Y on a 6+ hour flight. Especially on a flight where sleep is main activity.