It would be fine, if the updater (mac.informer) verifies the current OS-version before propose the update. Otherwise you have to expect many problems to remove the unmatched version and reinstall the old but working one.
Don't simply rely on download sites (MacUpdate, etc.). If you go to the developers site, they have versions of Onyx available for old versions of OS X as well as M/L. They say they're soon to release the Mavericks version as well.
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