How Would You Upgrade An Old Ox CNC Build Into A Serious Machine?

[JAZZCNC]

What you basically must NOT do is put any form of switch between the VFD and the three motor cables going to the spindle or unplug the spindle with the VFD running.

If you do you might hear a big bang.

Double up on that several times over!! NOTHING should come between the VFD and the spindle. Only ever use the spindle ON/OFF control wiring to stop the spindle if you ever want it to start again. I wouldn't even cut the mains input to the VFD with the spindle running, though that might be my clinical anxiety kicking in

Kit

While this excellent and sound advice, It isn't exactly correct.? . . . It's true that you shouldn't control the spindle ON/OFF with a Contactor between VFD and Spindle or use anything that will interrupt the connection while running, however, you can safely have a contactor between them provide it is not switched while the VFD is running.
I've done this before to control multiple spindles with single VFD but there must be safety involved and interlocks checking the spindle as stopped spinning and the RUN command isn't active before allowing it to happen.

This said the advice above is good.