Scenario 1: I have grandma's basic phone line eligible for an upgrade. Upgrade that line and get the S4. I will need to activate the S4 phone on this line (grandma's basic phone no longer works now). Go home and the very next day log in to My Verizon and put grandma's basic phone back on, using the switch and activate button. Then put the S4 to my line, therefore preserving my Unlimited! Go back to Verizon for a new SIM card.
Scenario 2: instead of paying full price of $699 or whatever, I would add a line (if you don't have five lines) and get the S4. The very next day, log on to My Verizon using the switch or activate button and change this new line phone to an old basic phone I have, so this new line costs me $9.99 per Month. Then activate the S4 on my line preserving my unlimited. Math: 2 years at $9.99=$240 plus $199 upgrade price or whatever it will be is certainly lower than a full $699 full retail price. Don't forget to go back to Verizon for a new SIM card! Do this on your own. I don't think Verizon can do this for you in the store anymore. You can order on line and pick up in store when it's released or maybe Verizon will have pre-order or something.