Yes, a backup which includes the following will allow you to build a fresh copy of OJS, either at the current version, or a later version:
- The OJS database
- The OJS private files (
files_dir) - The OJS public files (“public”)
- The config.inc.php
You can copy this backup to a new VM, either with the source code from 2.4.x, or with the source code from 3.x to have a new copy of OJS. Be sure to run the upgrade script if changing versions.
You will probably need to schedule a bit of downtime during the actual upgrade, but this can be on the order of minutes or an hour or so, depending on the size of the installation. You can practice this upgrade on another machine in advance to understand the time needed. If you have limited space on your production server (not enough for another full copy of OJS) this additional development server is strongly recommended. With limited space, you can upgrade in place, rather than upgrading a swapping in a copy, but if something goes wrong you will need to restore from backup, and this will take extra time.