Create a Test Site
If you haven't already done so, put your site into some version control system such as Git or SVN.
Then create a copy of the site on your local machine, so you can test the upgrade process before performing it on a live site.
I recommend keeping your drupal 5 stuff separated from your drupal 6.
e.g.
drupal-5/
drupal-6/
Each should have their own database, incase you want to revert back to the original.
Pre-Drupal 6 Install Step
1. Before installing Drupal 6, be sure to disable all 3rd party modules.
2. If you are using Update Status, be sure to uninstall it, as Drupal 6 has it in the core.
3. Change your theme to Garland.
4. Export/Backup all your Views.
5. Go into maint. mode ( optional )
6. I would backup the database at this point.
7. Adapt for your site & Run SQL Snippet below
NOTES:
* I had to drop the menu tables, because I was getting double of everything.
* I dropped the Views tables, you maybe able to get away with not doing this.
drop table menu; drop table menu_custom; drop table menu_links; drop table menu_router; drop table upload; alter table file_revisions rename to upload; drop table view_argument; drop table view_exposed_filter; drop table view_filter; drop table view_sort; drop table view_tablefield; drop table view_view;
Drupal 6 Install Step
1. Run update script to update tables, this update will probably take a while.
2. Run SQL snippet below
NOTES:
* uc_file only required if you use that module.
* if you did not delete the menu tables earlier you, probably wont need the second statement.
alter table `uc_file_users` change `key` `file_key` varchar(32) character set utf8 collate utf8_general_ci default '' NOT NULL; update variable set value = 's:13:"primary-links";' where name = 'menu_primary_links_source';
Drupal 6 Post Install Steps
1. Activate required 3rd party modules
2. Run Update Script
3. Recreate Menu if you deleted it earlier
4. Check Ubercart product settings
e.g. Disable Display Price http://example.com/admin/store/settings/products/edit/fields
5. If using Images feature in Ubercart, edit Product Content Types and look for Ubercart Image feature
http://example.com/admin/content/node-type/product
Check "Ubercart product settings", be sure you have your image field selected or no images will show.
6. Set the 'view own orders' permission on the authenticated users!
7. Recreate Views if needed
Thickbox Users:
* Check Thickbox Settings ( Disable Login http://example.com/admin/settings/thickbox )
Theme Adjustments
1. Ubercart renamed some of their CSS classes, usually all you have to do is replace '_' with '-' ... e.g. uc_price becomes uc-price
2. Checkout D5->D6 theme changes http://drupal.org/node/132442
Alright, now that you have practiced the upgrade process on a test site, do it again to be sure. Then when you are ready perform on the live site after you have made backups and have notified your customers that you will be performing maintenance.
If you are good the upgrade will take ~30min as long as you practice a couple times on test site. I recommend watching the logs for a while to pickup odd errors.