After a change of custody and termination of the original child support order, in which county do I file a motion for support?

I have 2 children and have recently appeared for a Change of Custody since of the the youngest child has decided to live with me. In the court order, it states, The Court declines to consider whether Respondent should be required to pay Petitioner (myself) child support for (the child who is now 17 years old). However, Petitioner is not precluded from filing a Motion that would seek such relief. I talked to The Child Support case worker and was informed that they were filing to close the original Child Support case now that my oldest child has graduated from high school, but they did not seem to know where I would file a new case for Child Support since I live in a different county. I live in Hennepin County and the Divorce decree and the original Child Support order are from Crow Wing County. How or where would I file a motion for Child Support? Thank You.