There are other extenuating circumstances that can be claimed to modify an order such as a material mistake of fact, fraud, undue influence, etc., but you are correct that generally there needs to be a change in circumstances before a modification will be granted. The fact that you and the ex came to an agreement suggests that you both made concessions of sorts in arriving at an overall resolution. T he fact that the court signed an order endorsing your agreement which is not perfectly correct on percentage of time (which presumably you both knew and nonetheless intended it be 50/50) is not ground, by itself, for the court to set aside your agreement and court's order thereon. Dept of Child support services does not have any sort elevated status in the case takes the case as is ----including with having a burden of proving changed circumsnces or some other legally exce ptable basis , there than she simply wants a "do over." I hope this was helpful and eases your concerns. Good Luck.
Please note that this answer does not constitute legal advice, and should not be relied on, as each situation is fact specific, and it is not possible to evaluate a legal problem without a comprehensive consultation and review of all the facts and court pleadings filed in the case. This answer does not create an attorney-client relationship.
I would point out to the court how much time you spend and file a request for a reduction.
My name is Stephen R. Cohen and I have practiced over 38 years and can be reached at 213-819-1171. I practiced mainly in Los Angeles and Orange County, California. I am not seeking clients from existing relationships with other attorneys, and give only limited advise over the phone (the phone is primarily used to set appointments), these services do not create an attorney client relationship. I apologize for mispelling< as I am a lousy typist, My answers may offend as I do not believe in pulling punches or sugar coating the truth. Further regarding courts in other states my opinions are largely based on logic and what I think is the modern trend which is to consider the needs of the child.