Dealing with production databases is always a risky process. Even if it's a minor upgrade from version x.01 to x.02, there is always a possibility that the database will stop working or data corruption will occur. Migrating databases also poses a data consistency risk. To avoid data loss, follow the instructions provided in this recipe, always use a test environment, and never delete backups until you are sure that the new system is working as expected, and you have a backup of your new system.
Before performing a database upgrade, make sure it is working with your Redmine's database drivers.
Follow the recipe Configuring Backup and Recovery to make a backup and take a snapshot of your virtual machine if possible.