You need to get a clear definition of what the customer expects to accomplish by doing the rebuild, especially what capabilities he expects. I don't know the details of what Siemens has to offer, but a couple of differences between CNC control and PLC-based motion control are in the coordination of axes and the ability to execute part programs written in G-codes.
When a CNC coordinates axes (called interpolation) the coordination is peer-to-peer. In PLC motion control, the coordination is master/slave. The difference is, if one of the coordinated axes in the CNC can't maintain the feedrate due to external forces, the other axis will slow down to compensate. In a PLC, if the slave axis can't keep up with the master, you get a fault indication.
A CNC will give you both linear and circular interpolation, the PLC may be limited to linear.
As has been mentioned, the customer should be doing a mechanical rebuild at the same time if he's looking to improve accuracy.