For the record, the only reason that you can't do the change online in this case is because the Web site won't let you do anything that could potentially cause complications. In this case, trying to trade phones online won't work, simply because the Web site will know that each of the phones is active on another line. Customer Service can override this to do both of the changes one right after the other, though. Definitely call Customer Service.
By the way, since your wording is a bit confusing, I'd recommend just saying that you'd like to trade phones between two numbers on your account. Think of each phone number as a container that holds your phone and all of its features. When we switch phones on an account, we're basically just taking each phone out of one container and putting it into another.