I have skipped 7 years of Wordpress Development (and was victorious over my laziness) and upgraded to the latest version. I must say that I have expected more troubles. As suggested I have done 9 incremental updates jumping over two major releases.
The routine was quite simple:
- Backup the database (one time action)
- Backup the wp-content/uploads (one time action)
- Disable all plugins
- Delete everything but wp-content, .htaccess and wp-config
- Download each major release from archive and copy it over the file system
- run /wp-admin/upgrade
- Repeat from point 3 until you’ve reached your release.
After all done I have even tested the direct upgrade from 2.1 to 3.9.2 and … it has also worked without issues.
Respect to the WordPress Team!