How to update modules to Drupal 8

Once Drupal 8 is released, there's going to be a huge gap of popular contrib modules. What do we need to know to update these modules to Drupal 8? What resources can help us get up to speed?
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Religious Drupal developers have a lot easier time compared to atheist Drupal developers, because they have no issues with form_alter! -wylbur

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