Can Big Sur be upgraded to Mojave?

If you're asking about upgrading Mojave to Big Sur, it depends on whether the Mac is compatible.

If you're asking about downgrading Big Sur to Mojave, the computer needs to be at least as old as Mojave, and you'll need to have either a Mojave installer, a Time Machine backup of Mojave, or a bootable clone of it.

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Keep in mind, downgrading you may not get all your Apple application libraries to transfer.

For more on downgrading see this tip:

https://discussions.apple.com/docs/DOC-1948