7.6The melody can be a chord tone or a non-chord tone
This is crucial.
Suppose harmony is C major:
C E G
and melody plays:
D
D is not part of the C triad.
Is that wrong?
No.
D can be:
- passing tone
- neighbor tone
- suspension-like note
- extension
So you do not need to change the chord every time the melody leaves the chord tones.
This is one reason harmonizing by “every note must belong to the chord” often sounds stiff.