Are you meaning the xml file? The post processor is used to tell a CAM package what G and M codes Etc that your controller can use.
If you are indeed meaning the XML then No Mach2 one cant be used for Mach3. You can however install Mach3 on the same computer as Mach2, then you can open Mach2 and note all of the settings for things like ports and pins, motor tuning etc. You can then close Mach2 and open Mach3 and insert these into Mach3. Mach3 layout is different but you should be able to follow reasonably well, if you dont get Mach3 working properly at first its no problem as you can then revert to Mach2, the only thing you CANT do is open Mach2 and 3 at the same time on the same computer.
Hood