I just extended mines a few weeks ago, very easy over the phone and they’ll send you out a new insurance letter to cover the extension.
Yes you can change at any time within the extended period but it’d need to be a much better car for little or no deposit to make me change…. the way I see it, for me it makes no difference if my car is 3, 4, 5 years old or brand new… I’m still paying the same money each month for it and all the costs are covered (servicing, insurance, tyres, repairs, etc) the only thing is I’ll have paid out a lump sum (£1,499 for the one I was considering) to have the latest reg plate? If I wanted a different car (shape, size, power, colour, doors, economy, EV range etc) then that’s a different story.
I also recieved my £600 GCB cheque and when I phoned to query this the said just cash it and if there’s more due after years 4 or 5 they’ll make up the difference then.