I know exactly what you want. I've encountered with the same thing before years ago, and It took me back to those days that maps were done with notepad when we didn't have map editors
Using notepad++ edit RSW (for ragnarok models) and GND files (for the ground).
Take a look at all the entries that ends with ".bmp" and make a note of the directories paths (the most of cases an official map tends to use only 1 directory path)
Use the find and replace tool of notepad++ by finding the directory you looked at and replace with your new name but PLEASE read: You can only change the last 3 or 2 letters of the directory you are looking for.
Lets make an example if you want to change a copy of all prontera texture models at the same time: open prontera.rsw and look for "ÇÁ·ÐÅ׶ó" and replace with "ÇÁ·ÐÅ×-1" (notice I changed the last 2 letters) save everything and now your prontera.rsw will read only that directory (which now you can copy with your custom textures) If you don't follow the same way (for example you edit the first 2 letters, instead of the last 2 letters) your map will crash, so be carefull and always do a backup.
Edit. By this simple method you can have multiple different pronteras with different textures in harmony as you said before. You can do the same with your maps.
