Bienvenu Benoît,
It's lot easier just to buy a 650 engine, it'll cost you lot more to transform the 550 into a 650. The carters are not the same, the piston sleeve is longer, the crank, the rod an maybe some other stuff. I would keep the 550 engine in case the 650 breaks don, then you just replace it and you can ride again while rebuilding the other one, that's how I do it.
Greets,
Michie