public class TaskOwnerHelper {
    
    /*
     * This will work for tasks under Accounts or Cases, just pass in those types of records
     */
    public static void syncTaskOwner( Map<Id, SObject> oldParents, Map<Id, SObject> newParents ){
        Set<Id> changedOwnerIds = new Set<Id>();
        
        // Identify all Parents that have a changed Owner
        for( SObject parent : newParents.values() ){
            SObject oldParent = oldParents.get((Id)parent.get('Id'));
            
            if( parent.get('OwnerId') <> oldParent.get('OwnerId') ){
                changedOwnerIds.add((Id)parent.get('Id'));
            }
        }
        
        // Find all the tasks (repeat for Events if required) that are not closed for the parent records that had new Owners
        Task[] restoreTaskOwner = new Task[0];
        
        // revert the owner on the found tasks
        for(Task tsk : [
            SELECT Id, OwnerId, Last_Assigned_Owner__c 
            FROM Task
            Where WhatId IN :changedOwnerIds AND IsClosed = FALSE AND Last_Assigned_Owner__c <> null
        ]){
            if (tsk.OwnerId <> tsk.Last_Assigned_Owner__c) {
                tsk.OwnerId = tsk.Last_Assigned_Owner__c;
                restoreTaskOwner.add(tsk);
            }
        }
        
        update restoreTaskOwner;
    }
}

@IsTest
public class TaskOwnerHelperTest {
	
    @TestSetup
    static void setup(){
        Account newAcct = new Account(
        	Name = 'Test'
        );
        insert newAcct;
        
        Task newTsk = new Task(
        	WhatId = newAcct.Id
            , Subject = 'Test'
        );
        insert newTsk;
    }
    
    @IsTest
    static void happyPath(){
        Task[] existingTasks = [
            SELECT Id, OwnerId, Last_Assigned_Owner__c, WhatId
            FROM Task
            Where IsClosed = FALSE AND Last_Assigned_Owner__c <> null
        ];
        System.assert( existingTasks.size() == 1, existingTasks );
        System.assertEquals( existingTasks[0].Last_Assigned_Owner__c, UserInfo.getUserId() );
        System.assertEquals( existingTasks[0].Last_Assigned_Owner__c, existingTasks[0].OwnerId );
        
        User[] usr = [SELECT Id FROM User WHERE IsActive = true AND Id != :UserInfo.getUserId() LIMIT 1];
        
        Test.startTest();
        update new Account(
            Id = existingTasks[0].WhatId
            , OwnerId = usr[0].Id
        );
        Test.stopTest();
        
        existingTasks = [
            SELECT Id, OwnerId, Last_Assigned_Owner__c, WhatId
            FROM Task
        ];
        Account acct = [SELECT Id, OwnerId FROM Account LIMIT 1];
        system.assertEquals( usr[0].Id, acct.OwnerId );
        System.assert( existingTasks.size() == 1, existingTasks );
        System.assertEquals( existingTasks[0].Last_Assigned_Owner__c, UserInfo.getUserId() );
        System.assertEquals( existingTasks[0].Last_Assigned_Owner__c, existingTasks[0].OwnerId );
    }
}