Backup is needed only if something goes wrong and you need to roll back to the older setup. Ideally you should just install new version on top and it should update your database structure and everything should work. I haven't done upgrades of versions with such big gap (3.7.1 - 3.9.1) and there is a possibility something could brake, so that's why it's good to have a backup. Also, the problems you can encounter could be not as a result of a bad upgrade, but maybe something changed in the code so radically, so things work differently in the newest versions and something will stop working as you expect. You should keep that in mind and do a test of all the main features you use after the upgrade, even if at first glance everything seems to work as before.
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