<p>See our <a href="https://www.bt.com/help/broadband/bt-cloud/getting-set-up/download-or-access-files/how-do-i-download-files--folders-and-devices-from-bt-cloud-">help page</a> for further information.</p>
The bit you're missing is that the careline units no longer use the phone line, so upgrade can be done no issues. Just need to persuade Openreach there's no longer any restriction on a flat by flat basis!
