Downgrade macOS 10.14 to 10.13 using Migration Assistant
Theoretical answer

This operation will fail.

Migration assistant is a huge procedure made to make all the conversions necessary to go from an older version of MacOS X to
the running one.

For example, the Mojave (10.14) version of Migration Assistant is configured to correctly convert the Users and System files from a High Sierra (10.13) set to a Mojave running environment.

The High Sierra (10.13) version of Migration assistant doesn't know the specific changes of Mojave (10.14) because they weren't known at its date of conception. Then this version cannot do a migration from Mojave to High Sierra.

On the other hand, the Mojave version of Migration Assistant does know the specific changes from High Sierra to Mojave but some of them might be not reversible, and this is usually not documented by Apple (this is bad: Apple is convinced none of its users is interested in this information).

Practical answer

Your proposed migrition might succeed (my raw estimate: 25%).

If you didn't made system modification or specific development on Mojave,

and if Apple didn't put in place a basic verification of the origin version of MacOS X, (is the origin version older than the target one?),

then you could try to run Migration Assistant from High Sierra to migrate only Users files from your Mojave backup to your running High Sierra environment.

Experience return

Please add your personnal success or failure as an answer, since this might help many other Mac users.