If the old disposal is an "in-sink-erator" style disposal (and most major brands of disposals are), then it's a quick and easy job because you don't have to change out the upper mounting components that connect to the sink.

You just disconnect the drain pipe, the dishwasher drain hose, and flip it over under the sink. Disconnet the electrical wires, then repeat the process in reverse to install the new disposal.

The one mistake I made (because I was in a hurry) was not to knock out the metal plug inside the dishwasher drain port. I ran the dishwasher later that night, and when the dishwasher didn't drain, I immediately knew what I had forgotten.