I don't know, GM has gone from Gen. III to Gen. IV to Gen. V (LT), the DOHC LT 5.5L, the Blackwing (didn't go anywhere), and a bunch of major revisions to the Duramax (with a brand new 8.3L on the way). Not exactly 'next to nothing'.
The 6.8L and 7.3L were not exacly 'all new' either, there's a lot of 6.2L Boss in their DNA...