I could have posted on the EE forum, but BT of course own EE and also Openreach although Openreach is supposed to be an independently run company, so that they are not perceived as being biased. BT are just using EE essentially as a residential brand so the question is still relevant to BT.
The telephone numbers are not specific to a particular supplier as they can be ported to any company (although it may not work in the BT checker). This has been the case for decades actually, although it was at extra cost years ago before more modern digital exchanges made the process a lot easier and at no cost now.
I'm not sure that there has been a change of equipment in the exchange, but of course the copper line used to operate via the Dslam which is adjacent to the cabinet. This will no longer be the case, so this may reduce the available speeds as there will no longer be any voltage (or not the to the same extent) on the copper line from the cabinet to the home. Actually the copper lines rely on current rather than voltage. They only operate at 50 volts, but quite high stabilised current. I was an engineer for 17 years, but I didn't really have much to do with the workings of broadband, I was connecting copper mostly. Without the copper connection the broadband speed will be determined by the ability of the broadband equipment to work over the full distance to the premises. As I'm quite a long way from the exchange, this may be why the speeds are not now available.