Thank you very much for the question.
There are obviously technicalities below that, but very simply, the standards that have been established under the versions of the TP312 document have gone in a series of updates over the decades. Each time those updates are established, obviously they affect the construction and operation of major components of an airport, including runways, etc.
The typical practice, and the practice that has been put in place in the most recent revision, version five, has been to allow for grandfathering and continuing of previous exemptions and previous standards under version four until such time as significant changes are made.