Hi Jon, and group,
I also belong to the Wooden and Classic Lightning Class Sailboat Group that ran into the same problem on Yahoo.
That group was successfully transferred to https://groups.io/g/classic-lightning
Not only the old treads but also all pictures and files. And from what I understand groups.io took care of the transfer.
So that might be another soution for the Sailwave group worth looking into.
Best regards,
Peter