Can I replace a WS-X6724-SFP
DFC3b with a DFC3c? If so, what is the part number? At this time, I
will even be willing to change the DFC3b to a CFC.
the DFC3C will support all the same features as the DFC3B. The 3C has a faster CPU and twice as much memory then the 3B. However if you have a Cat6500/Cisco7600 with both types of cards installed in it, the system will operate at the lowest common denominator it will be 3B.
Check out the below link for more information in DFC cards