This is a very simple and very usefull in improvisation and composition concept of chord-modulation , which is different from the concept of modulation by changinf the modes of the scale, and thus cahnging the melody.
E.g. Let a say that the melody are the notes x1, x2, x3 x4 x5 x6 over the chords A=(a1,a2,a3) and B=(b1,b2b3) with x1=a1, x3=a3, x4=b1, x6=b3.
We changethe chords A , B to A'=(a'1,a'2,a'3) B'=(b'1,b'2,b'3) . Then modulating the same melodic theme over the new chords means that we keep the pitch order relations and rhythm of the new melodic theme but we change the notes so that now it fits the new chords. Most probaly the a1 to a'1 , a3 to a'3 , b1 to b'1 and b3 to b'3.
This concepts allows to create nice melodies once a) the chord progression is predetermined b) an initial melodic theme has been created over a pair or triple of chords.
There are mainly 3 rules for the creation of melodic themes
1) Utlize 2 or 3 consecutive chords for none melodic theme
2) Make it connected by 2nds inside the main scale to give it naturality of singing
3) If one more of the 2 pr 3 chords are chromatic relative to the main scale make the melody pass from the external to the diatonic scale chromatic notes of the chords.
Here are 6 rules see also post 402
1) CHOSE THE TYPE OF CHROMATIC-TONALITY HARMONY
2) COMPOSE THE CHORD PROGRESSION AFTER THE CHOSEN CHROMATIC TONALITY
3) PARTITION THE CHORD PROGRESSION TO SINGLETS, PAIRS, TRIPLETS ETC OF CHORDS THAT WOULD SUPPORT BRIDGING MELODIC THEMES
4) COMPOSE THE SIMPLICIAL SUBMELODY (A DEFAULT IS THE MIDLLE NOTES OF THE CHORDS)
5) COMPOSE THE MOVE OF THE BRIDGING MELODIC THEMES BY THE TRIAD UP-DOWN-STATIONARY BY FOLLOWING PRINCIPLES OF BALANCE AND THE EMOTIONS OF THE UNDERLYING PARTITIONED SEGMENT OF CHORDS. ONE POLARITY FOR EACH PARTITIONED SEGMENT OF CHORDS.
6) COMPOSE THE RHYTHMIC PATTERN OF THE BRIFGING MELODIC THEMES AND THEMSELVES TOO.
