As with all the demographic groups, 0% of the Jews who voted for McCain in 2008 will vote for Obama in 2012. Any who switch from D to R this time around will be a gain for Romney. If that is only 10% it will be a big help. In Florida it could be enough to put Romney's margin of victory beyond the Democrat fraud capability.
Of course, you are working on the flawed premise that the same voters who voted in 2008 will be the exact same voters who vote in 2012. Obviously, there will be new voters who didn't vote in '08 and voters who did so in '08 who have since died or become incapacitated or who for whatever reason won't be voting this time around. In addition, some of the '08 voters will be voting in different states this year.