I migrated my FTTC connection a few times in 2010/11/12/13. On each occasion, the evening before migration day, I discovered I was 'dual' authenticated with the outgoing and incoming ISPs

Aquiss do state in their setup guides to use a specific login and password for PPPoE. Whether that’s actually needed or not (and generics may have worked) was possibly irrelevant for me coming from Sky, as they required the whole Option 61 client ID thingy - suspect that interferes with the normal settings for most ISPs. I removed that, set to use PPPoE authentication with credentials and it worked straight away.

Most Sky lines (over 80% and growing) don't need DHCP Option 61 and will pick up an IP via IPv6-PD (which has much more compatibility with 3rd party routers than Option 61).

Unfortunately that still wouldn't allow a seamless switch to another provider as configuring the new providers PPP credentials early would likely stop the Sky line working.