It's dependent on your speed estimate on signup. If it's less than 4Mb we provision on ADSL1 regardless.
This is because at speeds below 4Mb ADSL1 is often more reliable than ADSL2+, and below 2.5Mb you can often get faster speeds on ADSL1.
It's easy enough for us to change to 21CN at any point, and can be done by first line support.
@Jelv, we stopped doing like for like transfers ages ago.
Plusnet Support Team