Dennis, as said making sure there is a G20 at the start of the code would tell Mach that it is in inch units and it would run fine.
Grahams programme looks great as well and is probably the way I would go as I like to see the numbers I am used to. I looked earlier and its simple, download and run the exe, browse to the file you want to convert, give the new file a name, choose what units the new file is to be (mm or Inch) and then convert and thats it done, converted code has just been produced, load it and run
Hood