I believe most of it should be a straight sway, physically. You'll likely need electrical connections, fuel connections, and intake manifold (among others) changes to a 3rd and 4th Gen., but the Gen III and Gen IV are nearly identical on the exterior (except for things like cam, crank, and knock sensor changes.)
IMHO, this is the exactly reason I'm trying to wait for a top dog model, so I don't have to spend $1K on mods' that will void my warranty. Regardless, there isn't anything quite like the sound of a cammed V8 though.
05 Hummer H2 SUV on OEM Air Ride
Forged 11.1:1 408 LQ4 (K1 crank & rods; Mahle pistons)/CPP LS3 CNC heads/BTR Stage IV LS3 cam and valvetrain/home ported L92 intake manifold/ported 90mm TB/custom Volant CAI/mid-length headers/AFE dual 3" CB/Corvette Servo/el cheapo lift/Cognito UCAs/e'fan conversion/aluminum radiator/Moroso catchcan/HPTuners by me/LED exterior lighting/Pioneer AVH-5600BHS/Pioneer GM-9601/2-Pioneer TS-W310D4/Obcon Labyrinth dual-12 box/lots of other stuff