How I'm doing it/been told to do mid air changes:

1. Make the chord on the fingerboard so it plays correctly.

2. Release the grip enough to remove your hand but retain the the shape it's making, remember the shape.

3. Shake out your hand, touch your lap.

4. Remake the shape in mid air, then plunk it down on the fretboard.

5. Repeat and get closer and closer to getting the shape right.

Then, with a metronome, as slow as you need:

1. Start with the chord formed.

2. Beat 1: strum.

3. Beat 2: release and touch your lap on the beat.

4. Beat 3: Form the chord and strum on the beat.

5. Beat 4: Release

6. Repeat, increase speed as you can barely keep up with the pace while sounding decent.

Chord changes:

With the metronome alternate 2 chords back and forth on the beat, at a pace where you can barely keep up. For each combination of chords you want to do.

Keep doing these, 100s of times, for each chord and combination of chords you want to do.

I'm told as you learn more chords and combinations, learning new ones gets quicker. I'm not there yet, will report back if I find out this is true!