As the API is transitioning into using UUIDs, so are we. Don't worry, you will still be able to use your username, although using an UUID is recommended because I don't have to look it up then (but we cache it, if needed). We are still testing it, so you can't use your UUID yet.
Also, don't worry, we will provide an option to get your UUID, so you won't have to look it up.
So, in short: Your old urls will still work, but using an UUID instead of your username is a bit faster and will continue to work whenever you change your username.
Edit: We have now officially added support for UUIDs! We do not officially support usernames in the urls anymore, but they will continue to work. (But we recommend switching to UUIDs for performance)
I don't know my UUID??
Don't worry! You can just enter your username in the textbox as always! It will be automatically fetched for you, as long as you use the website to generate your signature.
So, what to do:
Generate your signature again here, and then use the provided link or BBcode. If everything's correct you should be using an UUID instead of username in the link, which means that you can change your name freely without worrying about the signature.