The reason for SDD being retired and Pathfinder being introduced is more because of the contract with Ford expiring, as the SDD is based on the Ford IDS, so JLR has to pay Ford to keep using it.
The major change to the cars themselves is the infotainment system, the Next Generation Infotainment (NGI) is an ethernet-based system, vs the older MOST fibre-optic infotainment i.e. the IAM2.1
So yeah, you could not (easily or economically) retrofit the NGI system into an IAM2.1 car, for example. The components and network are totally different.
But… the NGI has been in some JLR vehicles since 2016, and is supported by SDD. There will be a cutoff where new models are no longer supported by SDD and only by Pathfinder. But Pathfinder also works on older vehicles too, back to 2014MY (approximately).
The JLR VCI DoIP hardware interface is totally different and far more complicated than the older J2534 stuff. Mongoose cable is not compatible with it.
And JLR SDD is still being updated, was a new patch released a couple of days ago.
There is a considerable crossover between SDD and pathfinder, in coverage I mean.
The pathfinder covers vehicles back to 2014, so your car is covered into the future as well.