M999 would be something that Legacy has written, and I don't know what it does.
Normally, you would just change your Native Units and your steps/unit settings in Mach3, and you wouldn't need to do anything else.
Properly written macros should automatically adjust for metric and imperial, but apparently Legacy didn't do this with there M999.
Ask them to send you a metric version of the M999, so you don't have to do it yourself?
But I would still expect to need to change the Native units and steps/unit, but I don't know how Legacy configures Mach3 for their machines.