Xy500,
It's a matter of definition, yours may differ. The manufacturer designates the model year and it has very little to do with the calendar. If you chose to call yours an '07 because that's when it was manufactured and if your registration authority calls it an'07 because that's when you registered it so be it. So yes you have a WR250R that came off the assembly line 12/07 and was registered in your jurisdiction as a 2007. As far as Yamaha is concerned its a 2008 model year motorcycle because that's the model year they assigned to it.