I've just found this;
“HARMONIC INTERVAL”: JUST A FANCY NAME FOR “CHORD CHANGE” OR “CHORD PROGRESSION”
The term interval has a considerably different meaning in harmony, compared with melody. Simply put, a harmonic interval is a chord change.
A succession of melodic intervals is represented like this:
1 – 4 – 2 – 5 – 1
Each symbol represents a single note, called a scale degree. Each dash represents a pitch change from one single note to another single note.
So far, such pitch changes have been referred to as “intervals.” From now on, they’re melodic intervals, so as to distinguish them from harmonic intervals (chord changes). So, in the above example, there are five notes and four melodic intervals.
A succession of harmonic intervals (chord changes) is represented like this:
I – VIm – IIm – V7 – I
Each symbol represents a harmonic degree, commonly known as a chord. Each dash represents a harmonic change, from one chord to another chord.
Such harmonic changes are called harmonic intervals, or chord changes, or chord progressions. All of these terms mean the same thing. In the above example, there are five chords and four chord changes or harmonic intervals.
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There you go.
And there I go !