Seeing the alert of update, I downloaded WordPress 2.5, the latest version and upgraded this blog. I could do it without troubles, although I forgot to delete all non-configuration files of the old version.
Upgrading from any previous WordPress to 2.5:
- Delete your old WP files, saving ones you’ve modified.
- Upload the new files.
- Point your browser to /wp-admin/upgrade.php.
- You wanted more, perhaps? That’s it!