In one sense, divorce decrees are set in stone. You must do whatever is in the decree, and if you do not, your ex-spouse could seek remedies in court. In another sense, though, divorce decrees are not necessarily set in stone. The courts recognize that few people's lives stay the same and that they may need to make post-divorce modifications.
Of course, the legal process often requires time investment and the involvement of lawyers. So, if both you and your ex-spouse agree on a change, it is it really necessary to go through a process with lawyers and the court system again?
Situations when it may be necessary
Legally changing your decree is a good move in many situations, especially the more complex ones such as relocating with a child, significantly increasing or decreasing child support or spousal support and greatly expanding or decreasing parental access to a child. You and your ex-spouse should absolutely start to talk matters over yourselves if your communication is good. Collaboration is a beautiful thing, and the two of you being on the same page will speed up the process. However, legal modifications are necessary in case one of you changes your mind later or somehow challenges the new arrangement. If this happens, it is the original agreement, the one filed with the court, that applies to your situation.
For example, you and your ex-spouse agree that you can move out of state with the children. You do not modify the decree, and everything seems to be going well. A year later, the children's other parent decides he or she wants the children back in the state. The parent could argue that he or she did not fully agree to the move or sign anything legal. In the absence of a modified decree, the odds are much higher that you and the children might have to move back closer to the other parent.