How FullContact Migrated from MongoDB to Cassandra
This recent post from Michael Rose at FullContact details the contact management software's migration from MongoDB to Cassandra. It's a detailed account, starting with the background of what led to the decision - it wasn't a problem with MongoDB so much as a few early mistakes that grew over time - various attempts at solving the problems they faced, and finally how they went about migrating to Cassandra, including code and data to document their improvements.
It's a particular solution based on a particular set of problems, so it isn't a universal tutorial by any means, but there is a lot of value in such a thorough account of how one team worked through such a significant project. Check out Rose's full post for the whole story and some insight into how to manage database conversions.