Let's see,
8.C.2.a. Says a DH changed or cancelled by the company means the bank monies remain intact.
Don't remember if I've ever been in this exact situation, but, you should get to keep both banks. It's unusual, so might have to add it back into the expense report in FOX, not that tough to do.
Extra pay.
So, since if I understand your pairing correct, you had MEM-ABC-DH MEM, revised to MEM-ABC-DEF-DH MEM.
Sounds like one extra landing, and one extra duty period. Although a DH was canx'd, you still had a DH back to base.
I would request, .5CH for the landing, 3.5 for the extra duty period