I would probably do nothing in your position and change nothing as DV will come eventually to everyone even someone who is 110 and it seems to be moving slowly. If you change things or alter a BT package or resign a new contract or change provider DV seems to come a lot sooner. I am also dreading Digital Voice and we were told London was moving to it by end of 2023 but that has not happened.
Someone posted an online link https://www.bt.com/help/here-for-you above to register a person with additional needs so I suppose that might be worth doing.
If the people just have a landline and no broadband in theory once they are move to DV BT will send them a new modem (or in my case i have broadband too but my modem is old so they will send me a hub 2) and to start with the existing corded phone will plug into the back of the new modem. there will be another complications but that appears to be the essence of it.
