you can do it, but it's really a chore when done right. You'd be better off with virgin 35 Whelen brass. You'll need to expand the necks out to pretty much a strait wall case and then start necking it down in steps (probably at least one anneal when you get the necks back down to around an 8mm I.D.). When you get the necks down to 7mm you may want to anneal one more time. I would set the headspace about .010" too long, and the very carefully bring the shoulder down to where the headspace is correct. After that you'll have to fire form the brass and then trim it to the correct length. I can only think of one case design that would be harder at the moment, and that's a 30 Meyer made out of a .284 case. The 35 degree shoulder is a killer!
gary