CNC Lathe question

I have no experience with CNC lathes. How do control the long axis on
a CNC lathe? The Harrison I have access to has a gear rack and a lead
screw. Both of these have a fair amount of backlash which is no
problem in a manual environment but could be with CNC. The half nut
and lead screw have exceptional repeatability when threading but its
only in one direction.

When you convert a lathe to CNC do you replace the lead screw and
half nuts with a motor driven ballscrew assembly or do you just drive
the existing leadscrew and keep the half nuts? If you get rid of the
half nuts how do you run the machine manually?

Roger