The foundation is emailing contributors asking them to sign an agreement pertaining to the relicense.

We are successfully moving towards the next bug milestone which will put the new license in place going forward. This is likely among the most potentially disruptive steps as we need to ensure subsequent contributions are under the new license (additionally). We're targeting mid January pretty firmly now to make the cutover. We'll be sending a detailed message to the LLVM lists about this today.

And as the other responder mentioned, part of this has involved directly contacting all committers in the last two years (more recent committers more than once) to ensure we have everything we need to minimize disruption and make the cut over.