Your Mach3 set-up is just fine left as it is in mm.
If a Gcode file has been compiled in mm units then it should have a G21 at the start of the code.
If a Gcode file has been compiled in inch units then it should have a G20 at the start of the code.
Your machine set-up will not be changed by the G21 / G20 it is just the Gcode units that are converted during the program run. Unfortunately the roadrunner file was compiled in inches but they forgot to include the G20 (probable done just to fool us all
).
More details regarding this are in the Mach3 Manual
http://www.machsupport.com/help-learning/product-manuals/Once you are satisfied that Mach3 works to your expectations then order the license, it is perfectly in order to continue using Mach3 in demo mode for as long as you wish.
Tweakie.