Changing old CnC to shapeoko-like control system

I’ve got a Shapeoko 2 and I’ve been using it to death since the day I turned it on. However, it’s cutable area is bit on the small side and there are larger things I wish to build.
I acquired another CnC with a much larger cutting area on Craig’s list. It works, but it’s using Mach3 through a parallel port. I don’t particularly want to use a parallel port if I don’t have to and Mach 3 is far more complex than I believe it needs to be. How difficult would it be for me to replace the control system on this old CnC with the same system my Shapeoko 2 is using? (USB, GRBL, etc)

This might be a good solution for that application:

Hmmm . . very cool, and looks like a good option. I’m concerned, however, about how long the wait will be. The machine has already been sitting idle for two months while I try to figure out a solution and I’d like to get it up and running.
All that aside, this looks awesome.