A City of Heroes account started with a Sidekick code from the Deluxe edition functions as a standard game account, except that the trial period is 15 days instead of 30.

Do I lose my characters when the 15 days is up?
No, if you choose to continue to subscribe to to City of Heroes after the trial period, you can continue playing the heroes you created.

Even if you do not continue to subscribe immediately, innactive accounts are not currently deleted, so if you choose to reactivate the account at a later date, your heroes will be waiting for you.

(Under the City of Heroes character name policy, you may have to pick new names for your heroes if you leave your account innactive for 90 days or more, but you will not lose the characters themselves.)

Do I need to buy a boxed version of the game to continue playing after the 15 days is up?
No, you can continue to play City of Heroes simply by subscribing as usual, either with a credit or debit card or by using a Game Time Card. You do not need to buy a boxed version.

If I do buy a Deluxe edition box, can I upgrade my Sidekick account to the Deluxe version?
No, unfortunately it is not possible to upgrade an existing account to the Deluxe version. If you buy a Deluxe edition box, you can use the serial code to create a new account, which will have the Deluxe prestige power, badge and cape, but you will not be able to transfer your current heroes to it.