If the old amps/drives are working then I think I would keep them. They will almost certainly be analogue command so I would get one of the controllers capable of outputting +/- 10v .
There are a few, the one I use is the CSMIO/IP-A but there is also the DSPMC, Galil and Kflop with Kanalog.
Toolchangers can be done from VB using the I/O from the controllers mentioned above or using a PLC. I use a PLC on my lathe and VMC for two reasons, first I had them already, second I am better at ladder logic than VB script.
Hood