Trigger is : 

trigger TaskTrigger on Task (after update) {

          
          Map<Id,Id> ownerIds = new Map<Id,Id>();
        list <Lead> LeadList = new list <lead>();
        
    
    for(Task t:Trigger.new){
                Task TOld = trigger.oldMap.get(T.Id);
                String id = String.valueOf(t.WhoId);
                if(t.OwnerId != Told.Ownerid && id.startsWith('00Q'))
                  LeadList.add( new Lead(id = id, ownerid=t.OwnerId));
            }
    update LeadList;
    
}