Can We Modify

My step-daughter was in guardianship with her paternal Aunt and Uncle. They refused to release her back to my wife and I unless we had and parenting plan with her father that included joint custody and no financial responsibilities on his part. We begrudgingly agreed (verbal agreement with Uncle only) as fighting them could have cost us thousands on top of what we had already paid to file to get her back. We held up our end of the bargain and got her back in our home but now we are struggling a bit with finances. The parenting plan was final in Jackson County, Missouri last Oct/Nov. Can we now file to modify under the 20% rule as he pays no support at all or do we have to wait the 3 yrs to make any changes.