EmailHeader.triggerUserEmail doesn't prevent email notice from being sent

DMLOptions applies to DML calls made after the DML options are set. In a before-trigger context, the record update is implicit and not a separate DML call, so the DMLOptions setting has no effect on preventing the assignment notification email.

Workaround: Use an @future Asynchronous Method

The solution is to move the owner change logic to an @future method (an asynchronous Apex method that runs in a separate context). In the @future method, you perform an explicit DML update with DMLOptions applied, which correctly suppresses the assignment email notification.

The approach works as follows: the trigger collects the IDs of the Cases that require an ownership change and passes them to a @future method. Inside the @future method, you query for those Cases, update the OwnerId and reset the checkbox, then call Database.update with DMLOptions applied to perform the explicit DML. Because this is an explicit DML operation, DMLOptions correctly suppresses the email notification.

trigger ChangeOwnership_v2 on Case (before update) { 
List<ID> caseIDs = new List<ID>(); 

for (Case c : Trigger.New){ 
if (c.Change_Ownership__c == true){ 
caseIDs.add(c.Id); 
} 
} 

if(caseIDs.size() > 0) { 
CaseHelper.updateOwnership(caseIDs); 
} 
} 

public class CaseHelper { 

@future 
public static void updateOwnership(List<Id> caseIDs) { 
Database.DMLOptions dmo = new Database.DMLOptions(); 
dmo.EmailHeader.triggerUserEmail = false; 

List<Case> casesToUpdate = [select ownerid, change_ownership__c from case where id in :caseIDs]; 

for(Case c : casesToUpdate) { 
c.OwnerId='005i0000000QxRg'; 
c.Change_Ownership__c = false; 
} 

Database.update(casesToUpdate, dmo); // Actual DML operation is performed instead of implicit update
} 
}