tl;dr: If your app doesn’t store SoundCloud access tokens outside of the context of a user’s client (i.e. a database) the following should not affect you.
There will be upcoming changes to Access Tokens important for third parties integrated with SoundCloud. Previously, tokens were opaque strings that will be transitioning to a JWT format.
Ideally, clients should not rely on the shape/format of these tokens as this is transitive PII user data, but for example, embedded apps that are perhaps storing this information in a schema based environment, the size of the tokens will no longer fit in a basic 255 varchar. There will be no guarentees about size, as JWT’s are varying by nature and are dependent on the contents encoded inside of them.
Please update your app accordingly so that users do not incur any issues with your app’s integration.