Manually update 2017 iMac to Ventura; is backup needed?

In addition to the good guidance give by our friend, @Niel, I would also advise that before you install any upgrade, you create a bootable installer for the OS you intend to upgrade to, Ventura in your case. Should the upgrade go sideways, a bootable installer can be used to get out of a jam. All that is require is a 32 GB flash drive. Create the bootable installer and put it aside until it's needed.

Create a bootable installer for macOS - Apple Support

Apple does not provide a "rollback/revert function" per se, but one can take advantage of the Recovery mode to boot a Mac and reinstall macOS. This typically requires one to erase the drive and then reinstall the OS.

❗️Also, be aware, upgrading that iMac from High Sierra to Catalina 10.15 or anything newer - Ventura - will break all the old 32-bit software that is installed. Before you upgrade, download and use the free utility Go64 app from St. Claire Software to check your current apps and know what apps will need to be updated or replaced. Any app or printer or device driver that is not written in 64-bit code will not work in Catalina or newer.

Upgrading from the High Sierra can be done, but there will be some pain for those who have waited this long.

It is wise to ensure that you have a good backup (preferably 2 backups on separate media) before doing ANY upgrade. Things can and sometimes do go wrong.

Further, the jump from High Sierra to Ventura is an enormous jump and if the drive on the High Sierra iMac is formatted as MacOS Extended Journaled (which would have been normal on an iMac of that era) then it will also get reformatted to APFS either before or during the upgrade ... yet another reason to make sure you have good backups of your user data.

Also check all the apps you want to keep. If any are 32-bit apps you will need to upgrade them to newer, 64-bit versions, for which there may be a cost. You may need to upgrade certain apps anyway just to get versions that run on Ventura and in some cases you may have to purchase an entirely new license.

If it were me, I would upgrade first to Catalina due to the enormous changes in macOS that came with Catalina. If that works fine, then do a second upgrade to Ventura.

But in the end, consider that you would be attempting to upgrade a nearly 10-year old computer to a version of macOS that is already out of support.

1. Yes. Issues may be encountered at any time, not just during an OS upgrade.

2. Most backup methods are intended for personal information or media purchases, and don't include the OS; therefore, they can't be used to downgrade.

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