In a nutshell, grandfathering is used by all manufacturers to keep the initial applicable product specification:
- EASA TCDS EASA.A.064 for the A320 Family covers all types from ceo to neo;
- EASA TCDS EASA.A.015 for the A340 covers all types from -200 to -600 and I feel we can all admit that -200 and -500/600 are quite different A/C (a bit similar to 777X vs 777).
As stated by DaverReidUK above, we need to wait for the certifiaction to see if Boeing is able to do the same trick with the 777X.