The testing for upgrade/downgrade could use a generic testing framework.

What is envisioned for the first version of this is a basic test:

• Start mongod with a specified version and options
• Perform a setup function
  • Insert documents into a collection
  • Create a capped collection and insert documents
  • Create an index
• Stop mongod
• Restart mongod with a different specifed version and same options
• Perform verification function
  • Check number of documents in collection
  • Check the indexes
  • Check collection is capped
• Stop mongod