You've got to keep in mind that in any given POTUS election there are a lot new people who never voted before and just got into politics, many will never vote gain or not for years, and many will have major changes in ideology over 4 years. If you've voted R or D for 20 years you might still flip, but that's mostly D-->R in recent years, my party gets a lot of ex-liberals, and it's still nowhere near as common as flips when 18-30-somethings who voted once so far. We will get a lot of ex-Obama voters, Obama will get a fraction of that in Ex-McCain voters, and a lot of first time Obama voters will not return this time. Both he and Romney will get some brand new voters, Obama more then Romney probably, but of the sort who were new in '08 or who think 'off year election' refers to midterms, many of those who went for him in '08 as newbs will break GOP. I don't have a firm figure but I'd guess the GOP doesn't get 3-4% more defectors than the Dems but 3-4x as many... the Dark Side is seductive ... and we have cookies.

Normally the Dems carry about 1-6% more of the <30 vote then they carry of the country, see this chart, in '08 they carried a abnormally high 13% more, and 3 or 4% more of the vote then normal... that won't hold this time, they get jaded easily.