Well for starters you can only have one phone at a time on a single line. So, if I'm hearing you correctly you want to deactivate one phone and activate a different phone. If that is the mission then you can either simply call Verizon and provide them with the ESN# of each phone (usually found under the battery in most phones). It helps if you are calling them on a totally different phone from either of the two phones in question.
You can also do this yourself online via the website. Just follow the links for "activate" equipment and it will essentially give you the same instructions.