Move broadband from one phone line to another phone line at same address

[#1]

I currently have broadband on a second line originally installed when there was only dialup internet access about 10 years ago. Due to the ever increasing costs of this line I want to move this broadband account over to the main phone line at the same address and cancel the second line.
What is the correct procedure for achieving this move. Do I use the "Move my adsl" option even though the address is the same or is there another way.
My only concern using the "move my adsl" option is the possibility of something going wrong after the original line is canceled such as exchange capacity or problems with the new line.
Thanks for any help given.

[#2]

Have you investigated whether you could get the phone numbers on the two lines swapped over without affecting the ADSL?
I think moving the ADSL from one to the other would be the same as a house move - free if you haven't any deferred charges on your account and you stay with Plusnet for a year.