It seems to me that Rolls-Royce may have made modifications under the usual policy of continuous development that most technology companies pursue.
That doesn't mean that they believed an in-service failure likely, it may well be that their previous information showed that the older engines would reach their overhauls without problems. Imagine if they had decided to change engines unnecessarily, spending a lot of money without a good reason.
Now they have discovered otherwise and are taking action to correct this.