You can do it over the phone. I had to because someone got a hold of my number .......
Just call them and tell them you need to change your number. If it is the first time youve done it, i believe they wont charge you. It was free for me. You can tell them you are receiving harassing phone calls and need it changed.
And yes, got to call from another phone. It wont wipe your phone or change anything having to do with your root. I think they ask you to update your profile. Maybe prl while youre at it...? Just dont go in software update while youre in there. Lol