Thread:Project:Support desk/Upgrading from 1.15 to current, with no bureaucrat or admin/reply

I'm pretty sure the createAndPromote.php script exists in 1.15, so you could run that to make a new bureaucrat account that could grant admin rights to other users.

The recommended way to upgrade, in your situation, would be to use a new server (with recent versions of PHP/mysql) and install MediaWiki from scratch (a recent version) to a new database. Feel free to test some MediaWiki extensions and if everything works. Then you could take a backup of the old database and restore it on the new server, and make the new MediaWiki version point to that database, and then run the update script to perform changes to the database to adapt to the new version. You may need to copy the images/upload folder to get your uploaded files on the new server.

Note that even if you can't for whatever reason take a backup of the database and you only want the contents of the wiki, you can export the data in XML format and import it on a new installation, as explained in Manual:Importing XML dumps.