dal987, Are you sure it worked the way you wanted it to in 9.0.0.0?
While several things have changed between 9.0.0.0 and 9.0.0.8, I don't think there has ever been a version of Opus which wouldn't either open a new window or a new tab when you double-clicked a folder in another program.
Unless, that is, you edited the registry to change the commands which get sent on double-click. There were some threads about how to do this in the past so maybe you followed one of those and forgot about it? That may explain why things changed when you updated (the registry settings would've been reset to normal).
Anyway, Ken's taken the right approach as it'd be best to have a proper option rather than hack about with the registry.