I just helped a customer complete this type of conversion.
We used his original transformer (80VAC secondary) to feed the Centroid rectifier (CAPBRDLO assembly).
We used the 12VDC from the All-in-one unit for input power (limit switches and E-stop switch).
We changed the original "home" switches to normally-closed contacts, so they could serve as limit switches as well as home switches. Centroid/Ajax requires separate switches for plus and minus directions; can home off of limit switches; and requires that these switches be normally-closed.
We used his original contactors for emergency stop, spindle forward/reverse, and flood coolant.
We used his original motors and 1000-line encoders. Precision would have been slightly better with 2000-line encoders, but the original ones were good enough for his purposes.