15.37Reharmonization by modal interchange

Suppose melody is originally harmonized:

C | Am | F | G | C

Try replacing:

F with Fm:

C | Am | Fm | G | C

or G with B♭:

C | Am | F | B♭ | C

or Am with A♭:

C | A♭ | F | G | C

Not all versions will fit the melody.

The point is to experiment.