If I've signed the Canonical Contributors agreement, do I have to sign the new Harmony Agreement?
From the www.canonical.com/contributors/faq FAQ:
If I signed the old agreement, do I need to sign the new one too?
No, if you previously accepted the Canonical Contributor Agreement 2.5 (or earlier), your future contributions will continue to be covered by that. However, if you wish, you can sign the new Contributor License Agreement, in which case the terms of the new agreement will cover any new contributions.