Switching to a new but compatible license for new releases and contributions is different from relicensing all existing contributions.
If your project’s existing license is compatible with the license you want to change to, you could just start using the new license. [...] Essentially, with a permissive license the project’s copyright holders have given permission in advance to change licenses.
I've also written to SoftwareFreedom.org foundation to ask if that's ok.
This fixes #18.
After Open Source Guides, Legal - section 6: What if I want to change the license of my project?
I've also written to SoftwareFreedom.org foundation to ask if that's ok.