Will painless upgrading from Drupal 8 alpha 3.x to beta be possible? I'd like to create a website with Drupal 8 Alpha 3. But I'd like to know for sure if it's possible to do a harmless upgrade to Drupal 8 Beta once that it's released.

I heard something about the Drupal 8 API being frozen, but after reading this article (http://buytaert.net/drupal-8-apis-are-freezing-but-not-frozen) i'm not so sure anymore.

Can anyone help out?

PS: The website i'm going to build is fairly simple (few content-types, views, blocks etc)
No.

Upgrading from a previous 8.x version will be supported starting with the first beta.

Upgrading from 8.x-alpha will break, I know of at least one issue that will make major changes to the entity/node storage.