The latest information I got from Motability – admittedly just before the lockdown – was:
Any customer can extend the lease of any three-year-car now by up to two years and irrespective of AP.
The Good condition bonus increases too.
£600 after three years.
£700 after four years.
£900 after five years.
After five years, no dice unless the replacement vehicle has been ordered.
If one decides to terminate a lease extension, the GCB is paid out on whole years only.
A quick search of the Motability web shows different information.
I would call and get MOp to email the options.