While it is difficult to stop a modification process, it is possible. I would find out which Referee is assigned to your file. Tell them, or their staff, about the timing issue. They should be able to tell you whether it's worth your time to put something in writing and send it to them. They (FOC) will definitely want something in writing.
It also depends on the mother of the child. Even if you are successful in convincing the Referee that it makes no sense to modify child support when you obligation will terminate in less than 75 days, there would be an attempt to contact her and ask if she objects to the modification procedure being dropped.
In order for the process not to continue after the child is 18 and graduates high school, you need to write to the appropriate person at the FOC at least 30 days before the termination date. I would wait a week and then phone and ask if they got a copy of your letter. If yes, ask the exact procedure they perform to stop assessing support. If they say they don't have your letter, ask if you can fax it to them.
Elizabeth A. Silverman