I have a pair of DEs. They now need to be sent out for work, but that is a different issue. the DE as issued, isn't legal for CAS. It has that funky Bisley shaped hammer. If you have a DE or two and want to play CAS with em, you have two choices.
You can use Pietta hammers for a more traditional thumb piece. You can also contact Cimarron and use the snazzy lower wider hammer they have for their mounted guns. Both are pretty much "drop in."
You may however, encounter a problem with the firing pin bushing in the recoil shield. When bored/counter sunk for the conical firing pin, it was cut too thin and the firing pin wall owes out the hole till the primer flows into the hole and jams the gun.
Coffinmaker