There is a Michelin film that demonstrates what is very likely to happen: www.tyrereviews.co.uk/Article/Video-Can-you-fit-ju...m (TinyURL: tinyurl.com/72c6jrr)

If you give the front more grip than the back, then under braking in slippery conditions or when descending a hill, the back of the car will become uncontrollable. So while putting cold weather ‘winter’ tyres on the front only will give traction to get out of snow, it actually makes the car more dangerous to drive. Some insurers want to know you have fitted winter tyres, some don't. At least two (Toyota Insurance and Mercedes Insurance) give a 5 per cent discount for fitting winters all round.