Were Kerry president, nothing would have changed. The policies set into motion by Bush are presently proving extremely difficult to change, and Obama is far more liberal and devoted to "change" than John Kerry.
That's the best possible scenario. Worst case, he put a bad taste in America's collective mouth and Obama's chances of being elected were hurt. It may have even been called a desperate move, nominating a Obama after an unsuccessful Democratic president.
Writing this, I realize that Obama needed Bush, otherwise his mantra of "Hope" and "Change" would be called backtracking.