If you activate service with an existing phone you won't be tied to a contract, you'll then be eligible to receive the 2yr promotional pricing at any time.
To clarify, if I am on a family plan, can I add a new line without buying a phone (because I am using an old verizon-compatible phone), and later buy a phone at the 2-year contract price?
If I choose a month-by-month plan, and after 3 months, choose to switch to a 2-year contract and buy a new phone at the 2-year contract price, would I retain my number despite switching plan denominations?