Why couldn’t they just switch to the SD channel that already exists?
This is the obvious solution if PID switching was supported.
PID switching isn't transponder switching AIUI - it just lets you switch a service on one transponder between multiple video and audio streams (which have different PIDS) on the same transponder. Forcing a re-tune is a lot more complex (and I'm not sure how standardised that is as a function?)
These are differnt things. Having regional streams on a separate transponder involved requires code to be available on *ALL* Sky and FreeSat boxes, and a relationship established with a data stream that flags the transitions.
PID switching requres logic to be available at box level, but is much less invasive.
In the past Sky have vetoed both approaches, by requiring and eye-watering indemnity agaist degredations of platform stability.
The issue is that the Sky network is now nery stable, and very understood. If BBC sub-regional opt-outs were facilitated by having alternate streams, then would Sky come to the party?