Haven't played Diamond Throne OR A.U. yet, but I do have some thoughts:
The only thing I could see is that you may want to use the Races and Race levels in place of standard D&D ones. The classes REALLY don't play such an important role that they are inseperable.
Witches can be Bards (mechanics-wise, just make te bard songs hexes instead, and Perform requirements become witchery skill), Champions can be paladins without alignment requirements, and Greenbonds can be substituted by clerics, Magisters by Wizards or sorcerers, etc. It won't be a PERFECT fit, but the two things that need to stay the same are the Giant domination of the world, and their relationship to the other races. The other races can even be substituted (half-orcs for litorians, elves for verrik, dwarves for sibbecai, gnomes or halflings for Faen, etc.) But the Giants are the central campaign setting feature, IMO. The other races are secondary, and the classes the least important part to emulate.