Divorcing someone because of a change in religion brings up two points at once. The first is that they should divorce because a marriage between people with different beliefs doesn't work. The second is that he should find divorce acceptable because he no longer believes in a religion that says divorce is unacceptable.

Assuming we're talking about the second, then a scenario that does not involve a change in religion would be something like "I didn't realize it, but my religion says God is okay with divorce after all". That's implausible without something else going on, but it's possible that his religious leaders changed their minds, or that he misunderstood some points of his religion (for instance, perhaps his religion doesn't consider a secular divorce to be a divorce, and finds those acceptable). I would say that under those circumstances, yes, he probably would be okay with divorce.

So the answer is yes, you can break your promise.