If this is a racing style bike you can easily run into tire clearance issues because the smallest commonly available 27" tire is 1 1/8" wide, or about 28.6mm. Combine this with a 4mm greater rim radius and you can see how a lot of bikes that came with 700c wheels will not fit 27" wheels. But if you have adequate room, go for it.
OTOH there is no downside to using 700c rims on a bike that came with 27" rims except for the brake reach.