As I've stated in the other threads, I think his numbers will probably go down if he's not playing 81 games at Fenway Park, and the Yankees would be losing the draft picks to the Red Sox.
It would be an upgrade, but i'm pretty much against this. I can see why they're doing, but I think there are better ways to solve the first base issue (a platoon of Betemit and Duncan would be fine by me). There is no doubt Lowell is a solid offensive player, but at his age, and considering he's leaving Fenway, I'm not sure if he'd be able to keep it up.
EDIT: And considering how much they've spent on A-Rod, Posada, and possibly Rivera, it would probably be better to just go with the in-house options at first base.