Unlike child support, spousal support is often very difficult (if not impossible) to modify after the divorce is final, so getting it right the first time is a must!
I think it depends on what your decree states. If it says both parties forever waive the right to alimony, then you can't go back and revisit it. If it says it's reserved then that means they didn't make a decision on it yet and it might be awarded if you go back to court.