Yes, if you choose to buy the full version of City of Heroes, you can keep the characters you created in your free trial and continue to play with them.

When you create your retail account, do not use the ‘ADD GAME’ option on your Manage PlayNC Account page at http://www.plaync.com. Instead, use your retail serial code to convert your trial account into a full account.

In your ‘Game List’ at PlayNC you should see an entry something like this:

Click on the ‘ACTIVATE’ button on the right-hand side and you’ll be prompted to enter a serial code (depending on whether your trial account is still active or not, this button may say REACTIVATE). Enter your retail code and follow the instructions to reactivate your trial account as a full retail account.

Once this is done, you should be able to login to the game with your Trial account name and password. If you created a Message Board Account during your trial, you’ll also be able to continue using the user name and password you chose.