winter tyres radius

Hi everyone. I have a 2011 A5 Fwd 2.0 TDI with 19" tyres and alloys. The car is not lowered. The current tyres are Pzero summer tyres 255/35 ZR 19's. I want to get some winter tyres on for the alps but those in 19" are immensely expensive. Can I just get some new 17" rims and tyre setup with...