ULEZ - Disabled Car Tax Class

[sunnyb13]

My mothers car is sadly on its last legs and isn't ULEZ compliant, however there is a grace period until 2027 as Mother is disabled and her car tax class reflects this.

Can she replace her car with another non-ulez compliant car and still have the ulez exemption on it?

Looking on the website it's a bit unclear
[Quote]
Vehicles for disabled people
Blue Badge holders need to pay the ULEZ charge unless their vehicle meets the ULEZ emission standards or is registered for one of the following three grace periods (temporary exemptions).

1. 'Disabled' and 'Disabled Passenger Vehicle' Tax Class grace period
Disabled people whose vehicles are registered with the DVLA as having 'disabled' or 'disabled' passenger vehicle tax class benefit from a grace period which exempts them from paying the ULEZ charge until 24 October 2027. This is valid as long as their vehicle doesn't change tax class. Read the eligibility criteria for the disabled tax class exemption.

[/quote]

Does this mean

a) there was a static cut off period where disabled tax class cars at that point in time will be ULEZ exempt until 2027
or
b) any car that is or will be 'disabled' tax class will be exempt from the ULEZ charge until 2027.

[sunnyb13]

TFL don't seem to know themselves. spoke to them on the phone. they are unsure.