There is a nifty and inexpensive tool called a PVC cable saw that works really fast & easy. It is a hand tool, so I wouldn't recommend it for a big breeder who wants to change over all the water dishes all at once. But for a smaller operation or someone who is willing to do the switch gradually, it might be great.

My personal experience with a PVC cable saw is only in cutting smaller pipes, not big 4" ones and not in cutting fittings. However, it is supposed to be good for all those applications as well.