I think the two-phase approach is quite common where the phone line has to move as well as the broadband.
I had always had BT phone line, and the move from BE to Infinity was a direct swapover.
I have seen cases (at least I think I have, but can't cite one) where BT persuaded people to migrate to get the great benefits of Infinity,
then explained it had to be in two stages (via BT ADSL),
then once the user was on BT ADSL said "sorry, no Infinity available after all, but you are now locked into our broadband contract".
Moved (with trepidation) to BT Infinity 2 for upload speed. Happy BE user for several years.