Vendee:
Since the baseline Apache Guardian aircraft-engine interface will have been written around the T700-701D, the RTM322 will inevitably require a software mod. The Apache AH Mk1's EECU could be retained, but the software interface with the aircraft would - I believe - need to be updated.
For comparison with the D model, think of it in the context of the B-1B vs. B-52 conundrum: the Air Force has traditionally found it easier to add new toys to the B-52, since it can patch-on A/D converters to the 'analog' Buff, rather than having to mess with the core avionics of the 'digital' B-1B. Same with the Apache: since the original T700 in the AH-64D was a non-FADEC engine, the task of integrating the RTM322 - substantial though it was - at least didn't involve messing with an existing D/D AF/Eng interface.
The AH-64E is touted as using OSA, which in theory could help simplify things, but in general whenever a FADEC software mod is required engineers start weeping as if their favorite sci-fi show had been canceled.
As a sidenote, the E also incorporates a comprehensive IAC vibration monitoring system, covering engines, gearboxes, etc., with the goal of enabling conditioned-based maintenance.