When I converted to a Swiss license they allowed me to keep my old one. However, to ensure I'd never so much as think of using my Canadian license in Switzerland again, they had a very high-tech solution: they stuck a yellow sticker on my license that announced it was not valid in Switzerland (said sticker peels off very easily) and as an extra precaution, put an official stamp on the back of my card using what seems was a water-based ink... it had rubbed off the card by the time I got home from the licensing bureau.