We had a nice open session this afternoon at PyCon US, organized by Sean Harrison from Caktus Group. There were about 15 people in the room. The session agenda was generally to discuss improving Wagtail for developers.
StreamField data migrations
Adding fields or blocks won't cause problems as long as you don't rename or removing anything.
Sean presented a nice multi-step approach that he recommends when migrating StreamFields. This way you can run each step independently and verify that it worked correctly.