well when you start going up in size drastically like that, keep in mind that you're going to completely change the handling of the bike. With a 240 out back, you're going to lose a fair amount of cornering ability. How much? well it depends on the rider, really, but regardless, you will notice the difference.
As for installing a 200 rear tire, it will fit just fine, but you have to realize here that it's still going to be on the stock wheel, so it isnt going to be a huge difference. a 200 rear tire on the stock wheel is probably going to look about the same as a 190 on a rear wheel thats 1/4" wider.