P.S. I probably meandered too much and lost the main point I wanted to make.
Your 20/2.8 is equivalent to 13.5mm on DX. Assuming you need to sell the 12-24 to make the money work, I'm not sure that 1.5mm on the wide end should dictate the decision because you are gaining 2+ stops of high ISO performance, plus other modern features that have come along since the D300 (if they mean anything to you).
There is probably no point in trying to use the 12-24 DX at anything other than the wide end because you basically have 20-24mm covered in FX, in the context of what you get on DX now (it is 30-36mm DX equivalent). Although you don't have 24mm covered, you have 20 and 28 so 24 may not be so important. I am fairly certain you will NOT like the coverage at 24mm on FX - it won't buy you anything you don't have now (your 28/Ai).
You have a set of FX glass that many people would drool over. To me the main issue is the FOV shift you will get from your various lenses. Could be good or bad depending on how you feel about it. You shot film before DX so you fully understand the implications and have surely thought it through.