Software
How do I load an older OS version back on my MAC after upgrades that interfere with my software?
MacBook Pro 15″, macOS 11.6
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One can Only Downgrade to the Version of macOS installed on the computer when it was New.
Example: If computer came as New and came pre-loaded with macOS Big Sur macOS 11, that is the oldest version this computer could downgrade to.
Apples' Licence of Use will not allow downgrading to say Catalina macOS 10.15 or Mojave macOS 10.14.
That said - Do you have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup is very useful and can be used to Revert to Previous Working macOS.
Or the Nuclear Option.
Which will Wipe ALL DATA and there is no Recovery - period.
To perform this action will require booting from a Bootable Installer.
This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.
Once that is done read on for preparing the Destination computer
>> Only works on Intel Based Apple Computers.
Extra Special Notation regarding the Touch ID equipped Apple Computer
About Startup Security Utility and Must Enable from Recovery Mode the ability to boot from External Drive Before Attempting
1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.
2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive.
3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives.
4 - Choose the Upper Most Drive ( not the volumes indented and list below ).
5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map. This applies to macOS 10.14 Mojave and above.
5A - Formatting for macOS 10.13 High Sierra and below requires HFS Journaled with the GUID Partition Map
6 - Once that is done >> backup out of Disk Utilities and choose install macOS.
7 - Follow the prompts and it may automatically reboot several time.
8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.