ACF PRO uses a different structure than the free one. The database upgrade is used to convert the old field groups to the new one. If you’re worried about your live site, you can always clone it and create a staging/development site so that you can test the upgrade. Don’t forget to backup your site and database first. This page should give you more idea about it: https://www.advancedcustomfields.com/resources/upgrading-v4-v5/.
Also, the upgrade shouldn’t take long, so you can upgrade it whenever you want.
I hope this helps 🙂
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