...they would be righties.
You don't say Captain Obvious.
But results wise, serve-and-volley players like Laver and McEnroe would have lost a key advantage of their game and probably would have fewer slams, not taking a lot from them.
For Nadal, he would bully less the backhand but he's a natural rightie and one can assume his serve would have been more dominant. I guess he'd still have the same results.