I'm satisfied with this answer…
On the official Triumph site, they sell both the 8 and 7 plugs.
On the page for the 8 plugs, under Details/Technical Information, this plug is listed for use by
nearly 30 models, NOT including ANY Rockets: NGK DPR8-9EA
On the page for the 7 plugs, they list
ONLY the R3 Classic, Roadster, and Touring:
NGK DPR7-9EA
So, the 8 has been their go-to plug for many models for a long time, and they only deviate from it for specific reasons. On the 8 page, some of the models listed are only for particular VIN ranges. This shows they are detailed in their descriptions, and if they only meant for 7 plugs to be used in Rockets from a certain point forward, they would specify that. They clearly intend to switch to the 7 plugs in ALL Rockets.
Why? I haven't been able to find any definitive information on this from the Triumph engineers or secondhand sources, but based on the info in that NGK link above, there's a good chance they were seeing enough fouling to make the switch to a hotter plug.
My Conclusion: In this case it's not as simple as replacing the plugs that were already installed, as indicated in replies above. Triumph learned something that justified switching plugs, and while the 8s may continue to work perfectly well for many Rockets, I'm convinced it's worth making the switch to 7s in older models.