It depends on the type of account.

You can use a Standard or Collector’s Edition City of Villains serial code to upgrade any type of retail City of Heroes account, whether Deluxe, Sidekick or Standard (purchased online at PlayNC.com). Doing so will give you access to both games for one monthly fee. Your number of character slots will be expanded from 8 to 12, any of which can be used to create heroes or villains.

You can also use a Collector’s Edition City of Villains serial code to upgrade a Standard Edition City of Villains account (or a combined City of Heroes/City of Villains Standard Edition account). Doing so will give you access to the exclusive Collector’s Edition chest and cape emblem for any existing villains on that account.