At boarding time most GA's aren't going to want to mess with an "official" different class seat swap (hence why they told you it wasn't possible or whatever) but it should be easily accomplished once on board.

Just tell the FA what you're doing when you board, then ask whoever is seated next to your BF in economy if they'd like to swap for a nice seat up in first class so you can sit together. 9 out of 10 will happily take it.