While upgrading from UBB.threads 6.5.2 to the latest version, install/upgrade.php script gives this error: "Fatal error: Call to undefined function: upgrade_header()"
Ok, it's supposed to give you an error message. I tracked it down and it's a bug in the upgrade script. The error it's trying to trigger is that you need to do a full install of version 7 and then you import your 6.5.2 board into it. Because the changes were so extensive converting to 7.0 is a bit different from the past.
When doing an upgrade, you always want to make sure you don't overwrite: cache_builders/custom (unless you want to have no custom sidebars) includes/ (unless you want to loose all information reguarding your setup) styles/ (unless you want to b0g out your styles)