Thank you for your question and I look forward to assisting you. Please keep in mind that I can only respond to your post and the information contained in it, as I do not know what you know unless you describe it fully. Also, oftentimes there are future facts that are as yet undetermined in your matter, that can and must leave some areas of information provided by me broad in nature. However, don't hesitate to ask for clarification if needed! At times, there can also be a delay of an hour or more in between my answers because I may be helping other customers or taking a break. That being said...
You need to make a Motion to the Court that created the existing CS Order asap for a reduction in child support based on the new situation. If you have a lawyer, he can do this for you. If you'd rather represent yourself (pro se), you may do so and the Courts generally keep the forms needed for this very common type of family law motion - you need only stop in and pick them up. Be sure to read all of the directions (and the Courts generally have a nice packet of directions for pro se litigants, to properly follow the requirements of serving process on the ex, etc.) and you should have no problem - this is rather cut and dry, unless there are unstated other facts that come up.
Your oldest child's graduation from highschool COULD be basis for a change in child support (ceasing for him); you must look at your CS order that is currently in affect and determine what was agreed upon and/or otherwise Ordered by the Judge concerning when child support would end for each child. (Some orders continue CS through the college years, so it is very fact specific as to your particular order.) With regard to one of the children moving into your home, that is definitely a basis for ceasing CS for that child to the Mother who no longer will be providing him residence and, in fact, could indicate that you have the right to seek CS for this child from her. (In essense, it would have to be determined how her now owing you for this child would offset what you owe her for the now 2 remaining children in her care.)
Good luck with this, and I am glad you are delving into this in a timely fashion - I can't tell you how many payor-parents procrastinate and pay for 1,2 or 3 years at the original amount, simply because they can't seem to get themselves to MOVE! Go for it :)
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