When you open a database in a new version of the software, you are prompted to upgrade it. Once upgraded a database can only be accessed in that version of the software. If you re-install a previous version of the software, you will not be able to open the database anymore.
Also, if you upgrade a shared SQL database and do not have sufficient rights, you can cause the database to become unusable. Lastly, if you are able to upgrade a shared SQL database successfully, all other users must upgrade to the same version immediately or they will be unable to open the database.