You can swap it for a German one without taking a test.
More info here:
Getting A German Driving LicenseIn case you're too lazy to read the thread:
QUOTE (Malcolm Spudbury)
In the end, all I had to do was give them a photo, my passport and my old license, pay the fee (35 euros) and sign in the box. I didn't have to fill in any forms - they got all the info from their computer with my passport number - so I was in the office for no longer than about 10 minutes.
I got the new license about 4 weeks later. They took my old one off me, but I kept a photocopy of it just in case.