I think you'd find a lot of support to upgrading to Ethernet, rather than USB - there are some people here that have had problems with intermittent issues with USB in a high electrical noise environment. To this end, the admirable UC100 can be replaced instead with the EC300ETH, EC400ETH or AXBB-E devices from CNCDrive - works well with Mach3 and their own UCCNC software. These can plug directly (with suitable cable, of course) into the existing parallel interface that you have on your controller.