You did not indicate which version you have. In the MMM2014 manual, page 258:
One pitch higher/lower
You can quickly alter the octave of an included sample without having to access the
corresponding Soundpool folder in the Media Pool.
The "*" and "÷" keys on the numeric keypad (right on the keyboard) place the sample
one level higher or lower.
The "+" and "-" keys on the numeric keypad change the pitch via pitchshifting; the
respective effect in the audio effects rack is utilized for this.
In the first case above, it uses the other soundpool keys. In the second case, it uses pitchshifting, so this is what you want. When you do it, note the title change at the bottom of the soundpool in the arranger. It adds TS PS+1 or 2 or -1 or -2, depending on which way you go.
In addition to what browj2 said, and assuming you are using MM-2014....
For each soundpool object, there are also up to seven variations. In MM-2014, they are accessed using the column of numbered circles to the left of the media pool. (In earlier versions they may be a row of numbered bars above the media pool.) For melodic soundpool objects, you will have the same melody in seven different keys. For rhythmic soundpool objects, you may have seven different rhythms. Double-clicking on the numbered circle will insert that variation into your arrangement.
However, it will not change a major chord to a minor chord, a diminished chord or any other variation, unless the soundpools where set up that way. The best way to get want you want is to create your own MIDI sequence using the Vita MIDI Editor.