It'll slacken the bike, which will make it more stable on downhills but it won't climb as well, and the steering will be slower. There will also be more stress on the headtube...in all likelihood, not enough to be panic-worthy, but it may void your warranty. Assuming the bike was built around a 140mm fork, it won't be a big deal, but if the bike was built around a shorter fork on there, it may make the geometry and/or handling weird.