I have a Drupal 7 site with 40 content types. When Drupal 8 will be released than I would like to migrate these content types.
Would this be possible, or would I have to create this content types from scratch?
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I guess the best place to search for upgrade process is UPGRADE.txt file. Here you can find the step-by-step approach to follow in upgrading the site from Drupal 6/7 to Drupal 8.
Drupal 8 core will include a robust data migration API: https://drupal.org/node/2121299, part of the IMP initiative: https://groups.drupal.org/imp . Also you can check the recently updated list of modules that are included in Drupal 8 core: http://blog.merge.nl/2014/01/21/drupal-which-drupal-7-modules-went-into-drupal-core
Few of the links that you check:
Bas is right, but there is a lot of activity going on regarding this topic, so here is a more up-to-date (November 2015) answer:
Given the big difference in architecture between Drupal 7 and 8 there is no direct upgrade path, but Drupal 8 offers various migrate tools to upgrade sites from Drupal 6 and 7 to 8.
Using the core modules Migrate and Drupal Migrate and the contrib module Drupal Upgrade you can migrate all 40 content types from Drupal 7 to 8.
These modules allow you to migrate content types, content, comments, fields, vocabularies, terms and other Drupal 7 core entities and entity types (views, for example, cannot be migrated yet). Read about Drupal 7 to 8 upgrades.
Make sure you do not create any of the 40 content types before starting the upgrade. Read more about preparing an upgrade.
You can try to "upgrade" using the D8 core migrate module. Also install the contrib module migrate_plus. It allows you to convert custom content types, users, comments, taxonomy, etc. Most common fields should be covered. If you got something really custom going on, like a custom module, you'll also have to do some programming. But if you only used the essentials of D7, migrate might have you covered. In any case you'll have to rebuild a new site and migrate the content there, so it's not a painless solution in any sense.
Your most up to date documentation is found here: https://www.drupal.org/upgrade/migrate