since there is a certain period between the application for the kfw loan and its disbursement, the date of the application (we are talking about the kfw loan, not the building notification or anything similar) will be decisive 99% of the time. how else should it be done? they approve the old standard, you start immediately, and then everything has to be done all over again?!
as far as I understand, kfw70 will become the new building standard (less is no longer allowed) and kfw55 will become the current kfw70 (the "simple" eligible system without bonuses). there will probably also be new better standards that come with larger bonuses.
whether you want to build old or new must be calculated individually. in any case, building will be more expensive next year, as more has to be done especially in the affordable sector, since kfw70 will be mandatory.