To answer your questions, nothing is final until a judge signs it, so a person can attempt to reverse an agreement and go to trial.
However, as you are quickly finding out, one should never file a divorce without a lawyer, especially with a child involved.
Your son needs a lawyer at this point. Let me repeat it, because he can face huge problems without one. There are two reasons: (1) it is highly unlikely that two pro se parties who were unaware that an agreement could be changed possibly drafted anything close to optimal language, (2) even if the parties remain agreeable, a judge can reject an agreement, and (3) if a change is sought, a lawyer will know what legal arguments to make to try to keep the agreement in place (if in fact it should be kept in place).
The Parenting Plan, which is the document that outlines custody and visitation, does not become a final order until it is signed by the Judge and can be changed at any time up until then. Child support can only be set by the Judge and the parties have no authority to reach their own agreement regarding it.
If the divorce has been filed then you should already have a hearing date set for the Judge to review the paper work. Your son will need to be at that hearing. If there is not one set yet then he will need to set one down by Rule Nisi. If she is trying to change the custody visitation from what you agreed to then you need to tell the Judge that.
If you do not like the way the papers are written or you need to write new ones based on the fact that she might leave the State, then you need to do that and bring the original and two copies with you to court. But the most important thing is he must go to the hearing even though they have an agreement on paper.
The answer to your question is "Yes, before entry of a final decree, it is possible to change what is in papers initially filed in court in a divorce. However, your question implies that the "papers" included an agreement that your son and his wife already signed. Changing a previously signed agreement can be difficult but it is possible, particularly when the parties were "pro se" (unrepresented by lawyers) when the papers were filed. A final decree of divorce can be granted in Georgia in as little as 31 days so your son would be well advised to seek legal counsel quickly if he wants to change a divorce agreement he signed.
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