Ok guys, I have successfully changed any categories to my liking using the NAT program and one other technique. For every purchased library I have, what is easiest is to open NAT3, click on List and then the Add button. From here, add any programs you want to change. Once they're added, you can see the Cat, SubCat 1 and SubCat 2 for each program, along with the program name and description. All of this can be changed by clicking and typing in your change. Once you're done making changes to all of your added programs hit the save button and wait a moment for the process/overwrite to complete (basically no time for a few changes, maybe up to 30 seconds when trying to save half of HO's all you can eat - You can tell it's finished when you can use your mouse again). Then trash everything in your Nebulatemprepository temp folder and the ~scanboot in the Setups folder. Maker sure you've got Nebula set to scan your library when opening and finally... load a new instance of Nebula. The changes should be there.
I had two non-commercial libraries (the Botus Cabs and the new Revox Tape Machine library) and an old Cupwise demo that didn't work when doing this. The libraries simply disappeared from Nebula. The only way to get them back was to restore the original programs or rename the categories back to their original settings in NAT3. The alternate way of changing the category names worked for these programs. Simply open the programs you want to change in Nebula itself and hit the edit button. There you will see the three the category slots and you can change them by mouse clicking and typing the new category. Once you're done, save the program (sometimes a temp and ~scanboot trash and Nebula reload are needed).
Please note that one can't list "add" all of HO's all you can eat programs at one time in NAT3. I added half at a time and was able to edit them in two separate steps, closing NAT3 between two to reset the Added List.
All of this is somewhat futile without making and editing how categories are displayed using the Dictionaries Rom as mentioned previous in this thread and elsewhere by the many helpful Nebula masters!
Make backups before experimenting and Happy Organizing!