I would assume that if Kennedy dies no more than a year before that election, his wife will end up getting his slot. I very much doubt he is going to give up his seat willingly before his death, and I don't look for anyone to try to force him out.
There is no chance that he could live as long as the election of 2010, so that would be the last date for his replacement to be elected. Naturally no one will try to force him out, what would be the point.