This won't be a simple or cheap conversion. Most whats on front of the control console probably won't be usable as it will be bespoke to the FNC4 controller.

Like been mentioned the Servo drives are probably Analog controlled not Step/Direction which straight away makes it more expensive because there are not many Analog controllers available. They may even need higher Logic level voltages than usual to work ie: 12v or 15v compared to usual 5v most controllers provide(Cslabs units included)

Chances are the Servo motors are DC motors and most likely brushed not brushless and will/may have tacho. Changing the Servodrives is an option but won't be any cheaper because you'll still require Motion controller with lots of I/O and you'll lose some of the advantages Analog and Encoders give regards closing the loop back to the controller compared to Step/Dit that only close it between motor and drive.

Then you have the hassle of PID tuning the new Drives to Old motors along with tuning drives to controller rather than just tuning the Controller to the drives. Not always a simple affair and very important if motors are to perform correctly.

So what I'm saying is that do some research before diving into this and don't just let any old muppet take it on because it's very easy to get wrong costing lot of money and ruining a very nice machine.