All depends on what you have regards motors/amplifiers (drives)
The chances of them accepting Step/Dir input is close to zero, most likely is they need +-10v analogue input.
If you wish to keep the amplifiers then with Mach you have a few options.
CSMIO-IP/A
http://www.cs-lab.eu/en/index.phpDSPMC
http://www.vitalsystem.com/web/motion/dspmc.phpKflop with a Kanalog
http://dynomotion.com/Galil.
http://www.galilmc.com/Kflop will be the cheapest route in monetary terms but is a lot more complicated in the setup I would say.
Think CSMIO and DSPMC will work out roughly the same, maybe CSMIO slightly cheaper but depends on the exchange rate between, I presume, $ and €
Galil new would be the most expensive and would probably cost almost as much as getting new AC Servos and drives.
I have the step/dir version of the CSMIO controller on the wee lathe and really like it and yesterday I ordered the analogue version for the mill I am currently doing.
Hood