Like you said, downgrading would be a bad practice and is not at all recommended. Please make a backup of your database before continuing, just to play safe.
It looks like you’ve only replaced wp-admin folder. WordPress updates are not limited to wp-admin folder.
Try replacing wp-admin, wp-includes and all those files in the root directory.
Now clear your cookies and goto:
yoursite.tld/wp-admin/upgrade.php
WordPress might ask you to upgrade your database and continue with it. After this, you’d be running 2.9 (hopefully).
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