Upgrade from Drupal 6 to Drupal 7

Drupal 7 upgrade notes

• You may lose some string and variable translations in the process so you better get a dump of i18n_variable and export your locale translations for textgroups different than default.
• To upgrade from 6.x, first upgrade to the latest 6.x stable release. To upgrade from 5.x, first upgrade to 6.x, then to 7.x
• Other parts like the multilingual block system have been completely reworked and will need manual reconfiguration. Existing normal blocks won't be lost but the language settings will need manual reconfiguration.
• The module layout, dependencies and names have important changes, so it is advised to take your time, read the new modules descriptions, and decide on which ones you new enabled, that may be different from the Drupal 5 ones.
• Multilingual variables have been fully reworked. Now there's an UI to pick the variables you want to make multilingual, you'll need to manually reconfigure them.
• Internationalization modules now depend on the new Variable module

The upgrade process: Preparation

1. As with any other upgrade, make a full back up of your database before and set the site in off-line mode.
2. Disable all i18n modules (Disabling all contributed modules is always recommended) before upgrading to Drupal 7.
3. Upgrade your codebase and run the Drupal 7 standard upgrade
4. Review all Drupal 7 core language options, that have been really improved
5. Take care of any reconfiguration like moving old cck fields to new fields. If you run the i18n upgrade before doing this you may lose translations for field labels, descriptions, etc..

Upgrading Internationalization module

1. Go through the Internationalization package module list and enable the modules one group at a time respecting this strict order.
2. First enable the main Internationalization module and run the update script again (update.php)
3. Then enable String translation and Variable translation modules.
4. Review your string translation settings and multilingual variables visiting Administration › Configuration › Regional and language › Multilingual settings
5. Enable the rest of the modules you need. If you don't know what a modules is for, possibly you don't need it, so don't enable it until you miss some feature. Let the experiments with new modules and features for after the main upgrade.
6. Review again your multilingual settings and then your multilingual menus, vocabularies, extended node options, etc... Upgrade scripts don't take care of all possible combinations so you may need some manual reconfiguration.

Now if anything went wrong, you can file an issue. Don't forget to add detailed information about your set up before the upgrade and the problem itself.
(And by the way we are not interested at all on issues related with other contributed modules that are not at least beta releases for Drupal 7.)

For more information about Drupal Development, Drupal Experts, Drupal Developer and Drupal Programmer visit at http://www.dckap.com 

Source:  http://drupal.org