As far as I remember Kegworth is a case of mistakenly identifying the faulty engine,so what has to do with 'grandfathering' ?
Are you talking about the fact that a blade ruptured in one engine? So..?
As I remember,they incorrectly checked the vibration indicators (low is good as opposite to all other instruments ) ,and also helped by the fact that the vibrations reduced with reduced thrust,as they level off and desended, they decided to shut-down the good engine. After resuming thrust ,the bad engine failed,so they remained without engines,and landed short.
So ,again,what's with the 'grandfathering'..?