How to Improvise Over Chord Changes, Not Just One Scale
Playing one scale or pentatonic over an entire progression (see [[how-to-find-key-of-a-song:the article on finding the key]]) is a solid starting approach, but it has a ceiling: as soon as even one chord in the progression falls outside the key, 'one scale for everything' stops sounding right.
Inside the key, a scale works, but not equally well everywhere
Even when every chord in a progression comes from one key, some scale notes sound more 'at home' on a specific chord than others do (see the article on targeting chord tones). Mechanically playing the same scale notes regardless of the chord change already loses some of the available expressiveness, even though nothing is technically wrong.
A chord outside the key breaks that scheme entirely
A dominant seventh chord like G7 inside a key where it 'shouldn't be' (see the article on intervals) contains a note that's physically absent from the original scale - playing the original pentatonic over that chord produces a note that's clearly wrong, not just a 'less ideal' choice.
Think 'per chord', not 'per whole key'
The practical shift is to look not at the key as a whole, but at each chord individually as it arrives: what notes it contains (see the article on triads) and which of them are worth emphasizing right then. The key stays the general reference point, but the decision about specific notes gets made fresh on every chord.
Start with one 'problem' chord in the progression
Don't try to relearn the whole progression at once - find one chord that clearly steps outside the overall key, and work out on its own which notes over it are safe and which sound wrong. The rest of the progression's chords can keep being played the old way for now.
This is an addition, not a cancellation of everything already learned
The pentatonic and landing on chord tones (see the article on arpeggios) remain the working tool for the vast majority of chords in the vast majority of songs - this lesson adds an exception for a specific case, it doesn't replace everything learned before.
Try it in practice
Open chord trainer