There are quite alot of changes (around 9 months worth) to the various plugin components when moving directly from 1.5.1 to 1.6.8, so it's not completely surprising to me. The installer is really not designed to jump past multiple releases at all (probably better to uninstall/reinstall in those cases), so I suppose it's not too bad, as long as the plugin still loaded easily by the regular Rhino plugin loading method.
Thanks for the heads-up though, I'm just glad Rhino has a manual loading system available - if the SW plugin fails install in any way (it's much more complicated too), there's absolutely no way to manually load it.

JD