I have 4 active lines. But yes, you can easily switch phones online. I do it all the time. Especially while working on this article. For instructions how, go to http://sprint.com/landings/howtoactivate/index.html
The only thing is online activations only work between like phones. You can switch an account online from a LTE device to another LTE device, or a WiMax device to another WiMax device. But to switch from a WiMax to LTE device, or vice versa, you have to call Sprint.
Robert