There is absolutely no reason you can't change that from the top. There's a tensioner wrench you can buy at Sears or any auto parts dealer (Like MAC Tools or Snap-on tools). Basically, it's a long thin metal rod that you can attach a special socket onto, and it's made just for belt tensioners. It's only $40, and IMHO, it's worth it. I ended up stripping the bolt on the tensioner because I tried to get at it by removing the wheel, and the splash shield, etc., and it cost me $24 for a new one, and that was at my brother's parts discount!
Normally these are $70 at the dealer because no one makes an aftermarket part.
My advice? Don't risk chewing up the tensioner, and buy a tensioner wrench; but if you see that the tensioner is hosed, let me know and I'll see if I can get you a new tensioner for under $30.
