I tried the version in this post:
G-Code scaling program, mm to Inches and back it worked for me under Windows 7
If that file does not work you might want to register over there and ask Graham in the same thread. (looks like he is still very active there)
(If all else fails you can try G-Code Ripper: Open the g-code file and click on "Save G-Code Base". You will need to make sure that the "Plunge Feed" and "Z safe" are set to reasonable values but they will be used sparingly if you are only resizing. The default for G-Code ripper is to save in inches to go back to mm you would need to change the "Units" setting in the "General Settings Window".)
Scorch