Here's a Jam Track focussing on E & A chord changes.
Visual Jam Track 2 - Join In!
The aim for these changes is to be able to perform the change in a little less than a second. By doing this the chord shape is definitely becoming part of our muscle memory and thus our progress when strumming chords is accelerated!
Warm Up - Chord Changes
Try doing a single strum of each chord and changing from E to A and back as fast as you can!