Well, here's the official word on what makes an A-10C an A-10C.
The modifications are:
1) Digital Stores Management System (DSMS Integration - 5x7 display, CICU, CB Panel Mod, CICU SW, SubSystem Impact SW - SADL)
2) DC Power Generator Circuit Breakers
3) 1760 Bus Integration (cabling, circuit breakers, SCU A, Pylon, 1760 connectors, CICU Impact)
4) Digital Data Link
5) Targeting Pod Integration
6) JDAM/WCMD Integration