"Fight Night Champion will allow you to edit Licensed Boxers.
Small disclaimer: as I noted in the thread linked above, you still won't be able to edit actual appearance options (hair, body, etc.), but you are free to edit pretty much everything else, including ratings and boxer-specific AI tendencies.
I know many of you are probably wondering what this means for online. The only edits that will carry over into Online Ranked and Unranked are changes to accessories (which comes in handy if you want to beat people up as a pink-trunks-wearing-Tyson).
Fight Night Champion will allow you to create 100 CABs.
Small disclaimer: 3 of the 100 CAB slots are reserved for Online World Championship. So you can create 97 CABs that you can use in Fight Now, Legacy Mode, and Online Unranked."