Your characters are forever associated with the cd key under which they were created. This is done to prevent unauthorized users from accessing them. If you lose or change your cd keys and are unable to access characters in your vault (you are booted with a message on login), please contact a DM.
The following only affects a small number of users and may not apply to you:
If you play on multiple machines with multiple cd keys, you may need to figure out some way to maintain multiple versions of the nwn2cdkey.ini file in order to access characters locked to one cd key or another. This file is located in your /program files(x86)/atari/nwn2 directory (the exact path may differ on your machine). You can move or alternately rename versions of this file before launching the game, or launch the game with a batch file that does this for you.