The BOINC software (core client and all the server components) will evolve over time. There are a number of pairwise interactions in which version mismatches could cause problems:
Each version of the BOINC software has a major and minor version number. The client's version number is included in scheduler and file upload RPC requests. If a server receives a request from a client with a different major version, it returns an error.
Some changes to the BOINC server software may involve changes to the BOINC database (e.g. adding a new table or field). Such releases will include SQL script for modifying an existing database in-place.
Major-version changes to the BOINC software will require that all projects update their server software (and databases) and that all participants update their core client software on all hosts. This doesn't have to happen all at once, but in the midst of the crossover some clients won't be able to access some servers.
When a project makes a major-version change in its server software, it may need to create new versions of its applications. It must invalidate (by incrementing min_version) all app versions that are incompatible with the new server software.
When a participant updates the core client, all results currently in progress are discarded (because new app versions would be needed). The core client reads the version number from the old client state file, and discards the results.
TODO: figure out how this interacts with work sequences. Don't want to relocate a sequence needlessly.