FIELD_INTEGRITY_EXCEPTION, You can’t change the completion date on a case milestone that’s already exited an entitlement process.

I'm trying to write a trigger for Case that will reopen or uncomplete the Final Milestone in the Entitlement Process for that Case. I have run up against getting an error

FIELD_INTEGRITY_EXCEPTION, You can’t change the completion date on a case milestone that’s already exited an entitlement process.

I am using a method expanded from the milestoneUtils code found here:

This is my Trigger:

trigger openMilestoneOnReopenCase on Case (after update) {
    String finalMilestone = 'Solution Time';
    DateTime CompletionDate = NULL;
    List<Id> cases = new List<Id>();
    for(Case c:Trigger.new){
        //Get the prior update Case states
        Case oldCaseState = Trigger.oldMap.get(c.Id);
        // Checking if this is a Case being reopend from Closed
        if(c.Status == 'Open' && oldCaseState.Status == 'Closed'){
            // Get the Case Milestone List
            List<CaseMilestone> caseMilestones = [Select MilestoneTypeId, IsCompleted from CaseMilestone cm where cm.MilestoneType.Name=:finalMilestone AND cm.CaseId=:c.Id];
            //Iterate the Case Milestone List
            for(CaseMilestone caseMilestone:caseMilestones){
                //Check the Solution Time state isCompleted and add the Case to the Case List
                if(caseMilestone.isCompleted == true){
                    cases.add(c.Id);
                }
            }
            // Call the milestoneUtils Method
            if(!cases.isEmpty()){
                milestoneUtils.reopenMilestone(cases, finalMilestone, completionDate);
            }
        }
    }
}

This is the milestonesUtil Class I am calling the method from:

public class milestoneUtils {
    @future
    public static void reopenMilestone(List<Id> caseIds, String milestoneName, DateTime complDate) {
        System.debug('ASPD DEBUG: milestoneUtils: DateTime for completeMilestone is ' + string.valueOfGMT(complDate));
        List<CaseMilestone> cmsToUpdate = [select Id, completionDate from CaseMilestone cm where caseId in :caseIds and cm.MilestoneType.Name=:milestoneName and completionDate != null limit 1];
        if (cmsToUpdate.isEmpty() == false){
            for (CaseMilestone cm : cmsToUpdate){
                cm.completionDate = complDate;
            }
            update cmsToUpdate;
        }
    }
}