Do we have a case worth going back to court?

My fiancée got divorced almost three years ago. He was not represented by a lawyer due to funds. In their agreement his ex waived alimony in exchange for a larger child support payment. $2000 for two years then $1500 for two years. This agreement will end in February 2019. She does not have a job. She babysits children at her house and collect wages under the table. About a year ago her boyfriend moved in and we just found out that she is now expecting a child with him. We want to just get the child support payment reduced to what is required by the state. We think since she has no taxable income we can prove cohabitation. Curious if we have a sufficient enough case to get the agreement to end early and just have a child support payment.