Now that the second Drupal 8 release candidate (RC2) is available (with a significant fix for those upgrading from Drupal 6), it's time to start planning for your upgrade from Drupal 6 or Drupal 7. As I previously wrote, the migration system is "experimental" in 8.0, and at this time (mid-October 2015) the tools for upgrading are in a contributed module, Drupal Upgrade. The plan is that the upgrade path will be fully supported in core for Drupal 8.1 - to reach that goal, however, we need the path to be tested on a wide variety of sites in the meantime. So, we encourage everyone with a Drupal 6 or Drupal 7 site to start testing the upgrade path today - you don't want Drupal 8.1 to come out and only then discover your site has some quirk that's not fully accounted for. Even better, you might fight the upgrade path works just fine for your site and you can upgrade in the Drupal 8.0 timeframe!
The current documentation for performing your upgrade from Drupal 6 or 7 to Drupal 8 is on drupal.org - I'll walk through the process as it exists today, but should you be referring to this blog post after October 2015, be sure to review the official documentation for any changes.
This post explains running the upgrade process through the Drupal 8 web interface - if you'd prefer to use Drush, please see Upgrading to Drupal 8 using Drush.
