As King of Bulgaria, I usurped and then won the Title of "Emperor of Byzantium", yet my primary title is still the King of Bulgaria. Isn't an "Emperor" title supposed to be greater than a king title? Is there a way to change my primary title to Emperor of Byzantine by modding the saves?
No ... in the game, the Emperor title for Byzantium is the same tier as other King titles. It just has some hardcoded benefits that other's don't have. It also means that as Byzantine Emperor you can't release Kings as vassals. They will become independent.
Therefore King of Bulgaria is still your highest primary title.
But you can easily change this by editing your save game files like this:
To edit your primary title (supposing it is the Byzantine Empire) in a save-game do as follows
1. Open the save-game with Wordpad or Notepad
2. Replace all entries of BULG with XXXX (ctrl+h)
3. Replace all entries of BYZA with BULG
4. Replace all entries of XXXX with BYZA
Remember to make the replace 'Case Sensitive'. Do this by clicking on the box in the left corder when you have chosen ctrl+h in Wordpad.
Also be aware that switching titles will also switch all claims on those two titles.
So everyone who has a claim on King of Bulgaria title, will then have a claim on Byzantium title after the edit. And vice versa.
