Preparing the site
Complete the following steps to prepare your site for upgrading:
- Login as USER 1, the root user, the very first account created when Drupal was installed. If you cannot login as the root user, you will need to modify the update.php script once you load the new files for your site.
- If you are upgrading a Live Site switch the site to Offline mode :
(Drupal 4.7) administer >>settings
(Drupal 5.x) Administer >> Site configuration >> Site maintenance - Turn off (but do not uninstall) all modules that are not core modules:
You will be needing to get updated versions of these any way.
(Drupal 4.7) administer >> modules
(Drupal 5.x) Administer >> Site building >> Modules - Also, switch your theme back to the default theme ('BlueMarine' in 4.7 and lower, 'Garland' in 5.x):
(Drupal 4.7) administer >> themes
(Drupal 5.x) Administer >> Site building >> Themes

Taking the site offline
It isn't mentioned here. I'd take my site offline using the admin/settings/site-maintenance page.
Turing off modules
Note that this does NOT say to uninstall them. Just disable them. If you uninstall you will lose all the data those modules may have created.
There are a few modules that have not been, or will not be, upgraded - deal with them now. If a module is just not needed in the new version, follow the module maintainer's instructions on getting rid of it.
Also NOTE
that for D5 to D6, switching to Bluemarine or Garland (no other core theme) BEFORE making your upgrade is highly preferred, and may well avoid other problems. You might even want to make a second database backup after switching the theme.Turing off modules
Your theme might need certain modules, so you should switch your theme to a core one before turning off the modules – and after turning them back on again.
Don't loose Navigation
Make sure the theme you have choosen for upgrade has the standard Navigation block enabled if you have replaced it with a custom block that depends on a contributed module (such as DHTML Menus).
print a copy of the modules page
personally, i would recommend printing a copy of the modules page before turning off the modules which are not core modules. it's very easy to forget which modules you had on.
Raja Iskandar Shah
Consulting for Malaysian Business
www.norazilaco.biz
Or...
The Site Documentation module can produce an archival list on D5. But it would also be a good idea to figure out why a module you're not using is even there.
Nancy W.
Drupal Cookbook (for New Drupallers)
Adding Hidden Design or How To notes in your database