It's simple, (sort of) you add extra wood to the neck, right up under the part where the fretboard is extended and sand away what is now "excess" wood, then you sape the body joint EXACTLY like the AANJ on production models . . .
SOUNDS easy, but I wouldn't do it. I have an RG 550 from 1990 with the original heel and while I like the AANJ better, the old heel doesn't really stop me from playing just the same.
Steve Vai's "FLO" was originally a Floral Pattern Jem that was modified in this way (I don't think they added wood to the original neck, as "she" has a Jem 555 neck on her now, those are already AANJ) and painted white.
All FP models have the original neck joint, so there's simple proof that it can be done, and it works, but I'm sure it's more work, and more delicate woodworking, than an average DYSer should take on.
Personally, I say live with it as is, but if you REALLY want to do this, either have a pro do it, or be prepared to have a nice looking piece of firewood if you screw it up.
If you're talking about just removing some of the wood from the heel that's there now, not going for the whole AANJ shape, again, you have to be VERY CAREFUL. It's easy to remove too much wood and there's a lot of stress at that point of the assembly.
I tried to make a strat joint angled like on an original RG and ended up taking too much wood off for that particular guitar. Luckily strat bodies are everywhere and I just picked up a new one, but when I tuned the old one up and heard KKKKRRRRRRAAAAAAAAAAACCK!
, well, you know how I felt.