I retained my minibus and 7.5T entitlements at age 70, which involved filing forms D2 and D4. There was no official fee for renewing by this route, but I had to pay £140 in total to my optician and GP for thrm to do the necessary examinations and complete the paperwork. You need to book a 30 minute slot with your GP, and it took all that time do do the examinations & tests and complete the paperwork.
While your application is being processed, you do not have an actual licence but DVLA do (eventually) send you a letter confirming that you are entitled to drive all your original classes while your application is pending. As DVLA required additional medical tests (which they paid for on a private patient basis, no waiting months for an appointment), it took some 7 months before my licence arrived, but it was dated from its date of issue and not the date of expiry of the old one. I didn't bother at my next renewal and just did it on line. I had in the past made use of both the large minibus and the 7.5T entitlements, but deceded I wouldn't need them any more.
There was a special box on the D2/D4 forms to tick for those who wanted their minibus entitlement specifically for charitable/community purposes, which I didn't tick, but I don't know what the effect of this is. I think it might have been relevant if your licence did not have this entitlement originally.