I have a tub with a tub surround attached. Is it possble to remove the surround and replace it with tile or would I have to replace the tub as well as the surround?

You should be able to reuse the tub itself, you may have to tear up the surround to get to the screws that hold it together, but you may have another type that doenst have screws,
Once you get it removed just have the durock board lap over tub flange, it doesn't have to touch the tub, but maybe a 1/4 above the tub will be fine

It does not give any details on how to remove the tub surround... like do you rip it off or cut it and what tools are necessary... will it pull off the sheetrock behind it or just the paper layer?

You didn't ask how for one. Too now that you have ill tell you.
The surround is held in by sheetrock screws or nails in the nailing flange, to get to this you'll have to cut about an 1" of sheetrock out of the way to expose the flange , which is at the top of the surround. You'll need to do this to all 3 sides. And also down the front to sides as well.
Once you've done this you'll need to take a sawsaw and cut the surround out to the point that you can get to the bolts that hold the pieces together, maybe a foot up from the tub itself will do.
After you remove all the fasteners the surround should pull apart and away from the tub.
If you don't have a sawzaw to cut this with, there just 100 or less id get one, it makes this much easier, if you still don't want to buy that , then you need to use a skil saw, or just beat it with a hammer till it gives up. As far as the plumming just unscew the faucet and the flange plate, and also take off the shower head and pipe.

If you can not cut this out. You will need to remove the sheetrock off the wall and disconect the plumming so you can turn it in the room to reach all the fasteners in the back.
Good luck, this really isn't a tough job, you should be fine