Great.
Actually, just upgrading CB works fine with existing Groups.
It's the new setting Access Level that is still taken in consideration for registration, while it should not be.
This counts as the first bug found after CB 1.8 RC release. Congratulations! and thanks.
Bug #3245
Fix is in the tracker:
forge.joomlapolis.com/issues/3245
as patch or as file to replace.
Thank you!