I've read the books (ages ago), but I've never tried to use DFRPG for the characters in them.
What sort of stuff do you envision your players doing in an Old Kingdom game?
My first thought is that many of the physical powers in DFRPG might not be appropriate for the Old Kingdom. I don't remember many super-powered slugging matches in those books.
Well mainly the physical powers would be reserved for the powerful Free Magic Elementals and the Dead (lesser, medium and Greater). Many of those things displayed Toughness and Recovery, Speed and Strength. However mainly my players would want to be Charter Mages/Free Magic independent sorcerers thinking they can use Free Magic for good and Necromancers come wannabe Abhorsens. So essentially I am looking at the way I could do magic.
Charter Magic: Seems to be split into a few different areas:
Standard Charter Magic (Controlling the elements, making charter sendings, shape shifting things like that)
Wall Magic (a specialization of Charter Magic) allowing much greater enchantment and item creation.
The Gift of Sight: The Clayr's ability to see the future (a ritual or psyhic ability? it differently is of the Charter)
Then you have Free Magic:
Standard Free Magic: The provence of actual Destruciton (only free magic can directly annihliate something, you can destroy something with charter via fire but not "utterly destroy" like you can with Free Magic), summoning and binding, amongst other things.
Necromancy or Bell Magic: Bell Magic and Necromacner are essentially joined at the Hip but bell magic can be used on things other than the dead. Essentially Bell Magic/Necromancy is how one controls the dead but the bell magic can also be used to influence the charter/humans and free magic creature.
I am unsure how to approach this directly so far. I am still thing of the best way to go, ie wether to have Free Magic, Necromancy (Bell Magic) and Charter Magic be free seperate themes of magic (Like evocation or specific element evocation or ritual magic) and than have the other flavours like Wall makers be specializations within that area of magic.