Do no worry, circuits are being migrated to the 21CN, this is been going on for a few years now. If you are with BT Broadband, then nothing will change except you will be on the new 21CN MSANs.
Not sure what your concerns are. You will either have 20CN or 21CN internet, you will not be left without internet at all.
Where does it say anything there about it being switched off, I have never heard of such a thing?
BT are NOT going to switch off 20CN without implementing 21CN!! It will be one or the other.