downgrade from Montery to Big Sur

1. Create a bootable installer with Big Sur
2. Erase a Mac
3. Install from the bootable installer using How to create a bootable installer for macOS these steps (Use the bootable installer Section)

AND DON'T FORGET TO Back up your Mac

On the other hand, I would not recommend using an M1 mac with Big Sur.

Run this Terminal command

softwareupdate --list-full-installers What is the output ?

It should list all available Full Installers this computer Qualifies to run.

Should it list Big Sur 11.6.4 good

Terminal command

softwareupdate --fetch-full-installer --full-installer-version 11.6.4

In theory it will download the Full Big Sur 11.6.4 and will not launch but will place in Applications Folder a new Application called " Install Big Sur "

Now make the  Bootable Installer for Big Sur

About Startup Security Utility on a Mac with the Apple T2 Security Chip Very Important to be able to Boot from External Drive on T2 computer

As the article says, you should download the installer and quit it as soon as it runs. Then proceed with creating the bootable installer.

Well, only option for downgrade is a disk erase and bootable installer

Erwin_M Said:

"downgrade from Montery to Big Sur: Hi, I've upgraded my mbp M1, 2020 to Montery a while ago. Since then my battery is much faster empty[...]"

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Fro Reference:

Troubleshooting Mac Battery Draining:

Try a New Charger:

If it is charing from an external device, then use a new charger:

• Power & Cables - Mac Accessories - Apple - Select "Mac Compatibility" on the left
• Find the Right Power Adapter and Cable for your Mac Notebook - Apple Support

A Few Links to Refer to Power:

• If your Mac doesn't Turn On - Apple Support

• How to Turn your Mac On or Off - Apple Support

• About Mac Notebook Batteries - Apple Support

• Charge the MacBook Pro Battery - Apple Support

• If your Mac Battery won't Charge - Apple Support

Yes just be sure the USB Drive is large enough - usually minimum of 16 GB will be enough

Yes fine. Be aware the drive will be totally wiped of ALL data

You are welcome and thank you too.

Do come back to the Apple Support Communities ( ASC )  in the future, if / or when additional question may arise