I don’t know definitively but I suspect it would work. When I do an upgrade from main cabin to main cabin extra, it usually triggers the benefit.

My suggestion would be to go ahead and do it if you’re comfortable with that, and see what happens. If it doesn’t trigger it, you can call AMEX or do a chat and tell them that the charge was for a seat assignment. I wouldn’t refer to it as an upgrade, because I don’t think those are covered, but I think seat assignment selection is covered.