Best answer by David AKU
If I recall, you have to go into your account, and scroll to the bottom of the plan change page to change the phone line you wish to change, and then go back to the top (or reload the page) and implement the changes. So just make sure you have chosen the phone line you want to make changes, before you attempt to make the changes.
Right now the system thinks you want to make changes to the phone you have already adjusted.
Mobile Master Mayumi commented on this a few weeks ago, but I can't find her post just now.