Hi Detroiter,
I believe you can just install Mach3 into your computer. One or the other(Mach2 or Mach3) can be run, but not both.
While you continue to use Mach2, you can begin to configure your Mach3 and test in between jobs.
When you need the machine to run parts, you just use your Mach2 configuration/profile.
As you get Mach3 set up the way it satisfies you, you can begin to run the machine from Mach3.
You should be able to make a smooth transition.
John (Bloy)