Can anyone give me a quick summary on what extra work is required to upgrade a bridgeport servo (interact mill) cnc to Mach versus a Boss stepper Mach upgrade. I have done a Boss 5 stepper conversion to Mach and would like to do the same with this interact machine. I would like to use Gecko drives and and keep this similar to my stepper machine.

I cannot see how steppers are an 'upgrade' in any way from servos. I think you would be much better off staying with servos, even if you have to pick a different conversion

That's fine, but can anyone tell me the difference in parts and labor required between a stepper and a servo conversion, using a Mack / Gecko style retrofit

Well, if it is a complete interact, it already has encoders servos and tachs. You may choose to replace the servo amps, because the amps are not as 'stiff' as newer ones. If you don't want to, I would just look for a replacement amp to keep on the shelf as a spare. What you need is a cnc that will output +- 10v rather than step direction