Quite simple really. These steps were tested using Revit 2013.
- Select all masses, Group, and then Save as Library – Group
- Open the RVT from step 1
- R button – Export – Export Options – IFC Options
- Set Mass and sublevel Form to IfcBuildingElementProxy (copy-paste from another field)
- R button – Export – IFC
- R button – Open – IFC
- Your new file has all the same Forms, but instead of being Masses, they are now of Generic Model category. You can copy-paste them back into the original file, and either delete or hide the original Masses.
You could vary some of these steps to change the target category. But once you have the in-place Generic Models, you can switch them to basically any Category you like.