Once codes are created under a prefix, the format cannot be changed. You will need to continue with the custom format using the existing prefix or license a new prefix and set it up with an automated format.
You can download the Ad-ID prefix list here. The list provides all prefixes activated on Ad-ID as of the date of publication. You can also contact customer service who can search for a company and determine if they have a licensed prefix. Customer service will have access to the most up-to-date information regarding prefixes.