Sorry if this has been explained a thousand times over..searched briefly and didn't find what I was looking for.. I have an iPhone 5 - if I were to upgrade to a 5S or 6 - what would that entail SIM card-wise? For instance, if I purchase a 5S from someone - do I just pop my SIM into the new phone and it will instantly work with my carrier info or is there more to it? I thought in the past I had to go to Verizon's website and enter the ESN to activate it on my line, etc. - curious if I still need to do that and/or if there is something that needs to be done with the SIM. Thanks!