To really get a tune I've found that learning the melody really well helps. By doing that you see how it relates to the chord changes (ie this note is the xth of this y chord) and maybe take it through a couple of keys. Also simplifying the chord changes and turning them into numbers. Cycling a simple pattern through the chord progression (changes) so you hear the form.
In the end you should be able to play it with out a rythym section keeping the form.
Playing over changes has a couple of different meanings. In the most literal sense it means to play in the key center of the chord changes but most people mean playing the chord progression to the tune via outlining them.
you can do a lot with a tune by knowing all of the chords in the tune by being able to spell them and arpeggiate them at least through the 7th. remember that there are 5 basic 7th chords: major7, 7, minor7, minor7b5 and diminished 7 and these make up >99% of any given tune. if you know all these chords (5 types x 12 keys = only 60 chords) you will be able to play any tune.
likewise, playing the changes means that you can improvise melodies over the chords of a tune. this means creating melodic lines composed of chord tones on strong beats, scale patterns, arpeggios and approach patterns which are all derived from the chords.
check out my website for lessons on how to do this.
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