Yup, on x64 systems, changing from .dat to .exe causes error, and if run directly from iw4m.exe it gives mss32.dll error which cannot be fixed for some stupid reason -_-
EDIT!!! It seems I have found a solution for changing the .dat
If you used the autoinstaller, you need to dl the manual updater and install into your mw2 folder, then you can change it to iw4m.exe and run that and it works!
EDIT!!! IT seems it doesnt change the name anymore -_-