Have never done it of course but here's what I've read. First, if he has been on grain for a long time fat may have accumulated in the scrotum so that it will forever interfere with semen flow. Ok if you only have a couple cows and don't care how many times it takes to service them, but...
In the rumen, as I understand it, it takes different bacteria to breakdown different types of food stuffs so while corn bacteria is being reduced, hay and grass bacteria must be on the increase. That takes time so I'd agree that a gradual withdrawal is best.
If it were my choice, I'd stick to the original plan; eat him and AI the cows.