Some more information if anyone else is interested.
According to Pioneer it cannot be changed
http://www.pioneerelectronics.com/PUSA/Support/CarAudioVideo/In-Dash/CD-Players/ci.Can+I+purchase+a+radio+in+the+US+and+take+it+to+another+country,+like+Europe.SupportFAQ/?faqListingChannel=5f730c8a81ed4110VgnVCM1000002567a8c0RCRD
But I ran across this gentleman from England that was able to switch over a DEH
http://www.sparkgap.co.uk/misc/DEH6900UB_modification.htm
So I guess the first step is locating the service manual.