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