Complete standard upgrade steps of taking offline and disabling all modules and themes.
Run the ‘Repair’ SQL script agains the database.
Do the D6 to D7 core upgrade process documented at https://www.drupal.org/node/570162. Steps 15 and 16 are critical! So take backups of the sites folder and the database after Step 14.
Do Step 15 and be sure to take a backup. CCK does use:
Node Reference and User Reference so needs the References module installing.
Content Permissions so needs the Field Permissions module installing.
Field Groups so needs the Field Group module installing. Note the lack of the ‘s’ on the new module.
(NOT SURE ABOUT THIS? Start with the References module update and take a backup. Then do the References to Entity References update/migration and take another backup. This implies that the Entity References module installing. See the specific migration instruction page at https://www.drupal.org/project/entityreference_migration.)
Do Step 16 and be sure to take a new backup after each module update.
Next to do is the C-N-U theme. See https://c-n-u.eu/wp/2014/01/07/zen-upgrade/ and delete the C-N-U theme, upgrade the Zen theme and create a new C-N-U theme for D7 by copying the STARTERKIT folder up to the Themes level at sites/all/themes. It’s very simple to install following the README.txt file, especially as I’ve saved the normalize.css file from the sites/all/themes/cnu/css folder and the responsive.css file from the sites/all/themes/cnu/css/layouts folder.