I have an IID which worked great with my Westie but I see on the Gap website that they state "Limited coverage for 2017 and up vehicles" but with no other details.

Can anyone advise on what the limitations are? I don't want to pay to change the VIN code to my new FFRR if I'm not going to be able to use it. 2017 4.4 SDV8 Autobiography
2012 4.4 TDV8 Westminster - Gone, but not forgotten

Considering that the functionality list on their website doesn't cover anything beyond 2017 https://www.gap-diagnostic.com/products/iidtool/ I wouldn't hold my breath. There are many tools available that do the diagnostics and maintenance parts of IID tool, but CCF editing through a UI isn't necessarily one of their strengths...

The CCF editing is one of the key features that sets it apart from other fault code readers.

Did JLR change something significant with the MY17 electronics to make the IID features mainly redundant? 2017 4.4 SDV8 Autobiography
2012 4.4 TDV8 Westminster - Gone, but not forgotten

There are many differences introduced on the 2017-on models that limit the current IIDTool usability on these models.
The ECU interconnects are over other interfaces in addition to CAN Bus, for example automotive ethernet and flexray are also used.
The majority of diagnostic and programming communication with the vehicle is over an IP based interface known as DoIP (Diagnostic over IP). The ECU interconnects are over many other interfaces than CAN Bus, automotive ethernet and flexray are also used. The ODB (CAN based) interface is still available for fault code reading/clearing and monitoring most live values, you don't seem to be able to check the DPF fluid level with the IIDTool though.
As I recall CCF programming needs the DoIP interface along with the file having a completely new data format.