Not all, just some (from the Southern railways area mainly, like East Grinstead to Cambridge.
The change is to lengthen the ramp up period from one year to two years, since announcements of a transition period a couple of years ago the 24 tph was only achieved gradually over the course of 2018. Now it will be over a two year period. They had already intended to have future Thameslink services running short into London Bridge, Blackfriars or Kings Cross.
No doubt it will be reported on in some areas as a kind of disaster, but it could also be seen as sensible to have a gradual approach while they get to grips with the core area
ATO▸ and the changeover areas, and the wider area subject to Thameslink traffic management.
Suggestions elsewhere that the delay is about the only positive response to all those timetabling proposals in the "Gibb Report", most of which have been ignored as far as I can see...
Paul