In addition to renaming the file, you also need to change the name of the internal files using a Hex Editor (any free one will do). Open the file and do a "Replace" on the old name. In your example:
1. Rename arenas~0a_arena.big to arenas~pb_arena.big
2. Open arenas~pb_arena.big using a Hex Editor (again, any free one will do. I use
HxD myself).
3. Go to the Replace function. In HxD, it's in the Search menu or you can press Control-R.
4. In the "Search for" box, enter 0a_arena
5. In the "Replace with" box, enter pb_arena
6. Click OK
That should rename all of the files inside the .big file to the new abbreviation.