http://www.tyretraders.com/Tyres_Calculator.aspx
tyre calculator there, input 205/50/16 and 225/40/17 and you have 0.2mm out on the radius, which I suspect would work as thats the kind of difference you would get mixing new and old tyres on the front/rear axles, but, it would mean you need 2 new rims. I would suggest borrowing 2 wheels from someone just to double check before you parted with any cash.
I would also guess the sprung weight of the wheels would change so you might wanna play with damper settings on that axle.