Unless you want to write a Mach plugin that can communicate over serial, then you'll need to change/modify at least some of the electronics. You can either bypass the existing controller (i.e. whatever takes the serial input and converts it to step signals) and connect upto a suitable breakout board (you don't need to, but a BOB makes life far easier than trying to wire directly to a parallel port), or do a complete retrofit with new stepper drives and breakout board.
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It all depends on how much money you're willing to spend, and what drives are already fitted. New drives will give better performance, as the original stepper drives will most likely be very primative compared with modern drives, however it all depends on if the extra performance is worth it for you.
It also depends on if you want to go with a parallel port, or go for an external motion controller.