I'm in the process of upgrading an old installation of the Vanilla forum software, from version 3.3, to 2021.009.

Still running into some hiccups, but this's what's helped so far:

• I had to clear the cache directory.
• I had trouble with the 'pretty urls', as they've since become the default. My .htaccess file was also empty, so I copied the one from vanilla/vanilla/blob/master/.htaccess.dist. If you have a $Configuration['Garden']['RewriteUrls'] line in conf/config.php just delete it.
  • If you can't enable mod-rewrite then add $Configuration['Garden']['RewriteUrls'] = false; to conf/config.php. The links in the header will have to be fixed manually somehow, but I think the rest will work.
    • To finish the upgrade it will try to send you to [forum url]/utility/update, instead go to [forum url]index.php?p=/utility/update
• Apart from the cache, there's some other stuff that can be deleted:
  • container.html
  • plugins/EmojiExtender (there should be a new dir call plugins/emojiextender)
• themes directory has moved to addon/themes, and my old themes don't work.