Watch the Changes: Solo online guitar lesson by Jeff McErlain from Blues-Rock Evolution
Hipster lingo is as distinctive as Dickensian English. "Hey man, those are some hip changes" is to the jazzer as "Pardon me dear Sir, I find the sequence of chords you are playing quite fetching" is to Oliver Twist.
To translate: "the changes" refers to the chord progression, or chord changes. "Playing over the changes" means "to use one scale to solo over a chord progression." And "playing on the changes" means "to outline the sounds of the individual chords in a progression." ("Hip" means "interesting.")