Well, in case anyone else has a similar question, I just installed a UC100 on my lathe controller, and it was a doddle. XP computer seemed to have the .NET SW needed already installed, just ran the UC inslatter, opened Mach3, another couple of steps, and it was working. Apart from a bit of tweaking to the Z axis speed all is fine - all the port/pin settings are the same, the X zero, index pulse and probe all work just as before, as do the tool zeroing macros.
The Mill was still connected to the parallel port, so I fired that up, selected the parallel port when the dialogue came up, and it was operating normally.
So the answer to my first question is YES.
The second question though is open but maybe I'll post that in the WIN10 thread.