I do this all the time to get my colleagues moved up. I've never had a problem with it, even if there are no seats together anywhere in E+. They usually try to put them as close as they can and suggest asking someone to swap on the plane. I thought I'd read that preMerger, you could only move someone into the seat next to you. If so, that doesn't seem to be the case anymore.

I'm not sure it would work if I tried it after I'd gotten upgraded, so I always do it before my upgrade window opens. No one has ever been swept back into Y after I've been upgraded. The only issue is that they can get left behind in a middle seat. I didn't feel right suggesting that they put them in the aisle or window and me in the middle in case I got upgraded.