Without knowing what plans and promos you're signed up for, I'd air on the side of caution before you make the change. Some plan offers/promos require that you keep the line in order to keep that offer. Have our Care team look over your account before you make that change to be on the safe side.
i would contact TMO either through Facebook or Twitter..higher tier support and they should be able to answer your questions.
the only employees in here are admins/moderators for the site only..they do not have account access