Trigger to Update Field based on Formula Field

trigger projectbilltoaccount on Ticket__c (after update){
    for(Ticket__c newchange:Trigger.new) {
    Ticket__c oldchange = system.trigger.oldMap.get(newchange.Id);
    if (oldchange.Bill_To__c !=newchange.Bill_To__c) 
        {
          Ticket__c temp=[select id, Bill_To__c, UpdateRequired__c from Ticket__c where updateRequired__c=false];
          temp.UpdateRequired__c=True;
          Update temp;
        }
        }
}