Drupal 6 to 7 upgrade process tricks

Note: DO NOT run install.php at any time during an upgrade. It will empty your content from the database.

Upgrade Steps

1. Backup your existing site and database.
2. Update Drupal core, modules and themes to the latest releases for your existing Drupal version.
3. Log in as user ID 1 (the site maintenance user).
   This is the user name that you created during the installation process for your site.
4. Put your site in maintenance mode
   Go to the site maintenance page, select "Off-line" and save the configuration. If you have defined a custom maintenance theme in your settings.php file, comment it out before proceeding.
5. Change all themes to Garland
   Go to the Themes page, enable "Garland" and select it as the default theme. If you have been using a separate theme for administration, select "Garland" for your administration theme as well. If you are using a custom maintenance theme, comment out 'maintenance_theme' => 'your theme name' in settings.php.
6. Disable non-core modules
   Go to the Modules page and disable all modules that are not listed under "Core - required" or "Core - optional". It is possible that some modules cannot be disabled, because others depend on them. Repeat this step until all non-core modules are disabled.
   If you know that you will not re-enable some modules for the target Drupal version and you no longer need their data, then you should uninstall them under the Uninstall tab after disabling them.
   You can also use Drush to disable all non-core modules (first save a list of all enabled modules):
   drush pml --no-core --type=module --status=enabled --pipe > modules.txt
xargs -a modules.txt drush -y dis
xargs -a modules.txt drush -y en
7. Remove default settings file
   On the command line or in your FTP client, remove the file sites/default/default.settings.php
8. Remove all old core files and directories
   Remove all old core files and directories, except for the 'sites' directory and any custom files you added elsewhere. If you made modifications to files like .htaccess or robots.txt, you will need to re-apply them from your backup, after the new files are in place.
9. Remove uninstalled modules
   If you uninstalled any modules, remove them from the sites/all/modules and other sites/*/modules directories. Leave other modules in place, even though they are incompatible with Drupal 7.x.
10. Download Drupal
    Download the latest Drupal release of the target version to a directory outside of your web root. Extract the archive and copy the files into your Drupal directory.
    On a typical Unix/Linux command line, use the following commands to download
    and extract:
    wget http://drupal.org/files/projects/drupal-x.y.tar.gz
    tar -zxvf drupal-x.y.tar.gz
    This creates a new directory drupal-x.y/ containing all Drupal files and directories. Copy the files into your Drupal installation directory:
    cp -R drupal-x.y/* drupal-x.y/.htaccess /path/to/your/installation
    If you do not have command line access to your server, download the archive using your web browser, extract it, and then use an FTP client to upload the files to your web root.
11. Re-apply modifications to core files
    Re-apply any modifications to files such as .htaccess or robots.txt.
12. Make your settings.php file writeable
    Make your settings.php file writeable, so that the update process can convert it to the new format.
13. Run the update script
    Run update.php by visiting http://www.example.com/update.php (replace www.example.comwith your domain name). This will update the core database tables.
    If you are unable to access update.php do the following:
    • Open settings.php with a text editor.
    • Find the line that says:
      $update_free_access = FALSE;
    • Change it into:
      $update_free_access = TRUE;
    • Once the upgrade is done, $update_free_access must be reverted to FALSE.
    If your update script runs without errors and you can view the home page of your upgraded website but none of your links are working (page not founds when clicking on menus etc) AND you were using clean URLs, go to /?q=admin/config/search/clean-urls and turn off clean URLs.
14. Backup your database
    Backup your database after the core upgrade has run.
15. Upgrade fields
    If you are upgrading from Drupal 6 to 7 and were using CCK (and perhaps additional modules) to create fields for your content types, you will need to upgrade the data in those fields as a separate step. Download the Drupal 7 CCK module, and turn on Content Migration. Go to Structure > Migrate Fields or http://example/com/admin/structure/content_migrate for a page to walk you through the migration process. There are now several types of fields in core, but not every type. You might need to download Drupal 7 versions of contributed modules to support other types of fields (such as the References module for nodereference fields and/or userreference fields). Learn more about migrating content from CCK to Core Fields.
16. Update contrib modules and themes
    Replace and update your non-core modules and themes, following the procedures athttp://drupal.org/node/948216
17. Check the Status Report
    Go to the Status Report page and verify that everything is working as expected.
18. Make sure settings.php is secure
    Ensure that $update_free_access is FALSE in settings.php.
    Remove write permissions.
19. Check Drupal Core Modules
    Make sure that standard modules from a typical fresh install are engaged. In a Drupal 6 to 7 upgrade, some modules that may not be engaged are: 'Dashboard', 'Contextual Links', 'Overlay', 'Field', 'File', 'Image' and 'Shortcut'.
20. Remove your site from Maintenance Mode
    Go to the Maintenance Mode page and disable the "Put site into maintenance mode" checkbox and save the configuration.
Note that the file structure, particularly the locations of core and contributed modules have changed in Drupal 8.