One good exercise you can do is hold a chord for 2 seconds, then release while spreading your finger a little bit (so that you don't keep your finger into the chord position) wait 1 second then make the chord again. Don't try to change chord right away, do the same chord.

Do this for about a minute, then change chord.

Do the whole thing three times. (total 6 minutes, alternating one minute on each chord)

The next step is to hold one chord for 2 seconds, then change chord but keep it to one second. So it would go D (hold two second), release (one second), C (hold 2 seconds), and so on.

I'm sure with this simple exercise, you're switch will get better and better.

Let us know how it goes.