The district does favor SSM, Young will most certainly not be changing his position and does not need to do so, and is just as likely to be knocked out of his seat by an asteroid impact as by a challenger. I think you might be overestimating how much of a wedge issue this is.
Bill Young just had his closest election *ever* in 2012, despite his district being drawn to become (marginally) more Republican. Old congressmen who are personally popular have found themselves thrown out before after their districts changed enough. Not saying that this would be the issue that destroys him, but Bill Young's seat is not nearly as rock-solid as you make it out to be.
He's also hugely hostile to gay people in general, though, so he definitely won't be supportive of same-sex marriage any time soon.
