change from Ventura back to Big Sur

You would need to erase their computer entirely, and install Big Sur from a bootable USB drive.

click here ➜ Erase your Mac - Apple Support

then here ➜ Create a bootable installer for macOS - Apple Support

You can download Big Sur from here ➜ How to download macOS - Apple Support

TripleDigits wrote:

Thank you so much. You should make the bootable installer for Big Sur first and then erase your Mac.

Yes. Make the installer first.

This should work for an intel-based computer. I know I can download Big Sur onto an intel-based computer. I can't download it from Mac Studio because there, Ventura will not allow it. I am now on my way to installing Big Sur on my MacBook Pro. Thank you again!

You're welcome.

You can download it on any computer Intel or Apple Silicon, but if its already running Ventura, you need to have downloaded it at some point with the same Apple ID currently being used on the Ventura computer for it to appear as an option in the purchase history. Also note, that if the Mac running Ventura came with Monterey or Ventura already installed from Apple, you cannot install Big Sur on it. It will only support the OS it came with or newer.