I have a 84 Trek 660 that came with 27's and when I converted to 700's I didn't have to do anything. Back in those days "MOST" bikes were sent out with medium reach brake calipers that had enough movement in the pads to fit either 27 or 700.

But as others have said 27' tires are readly available but you can no longer find highend racing tires or even midlevel training tires (Conti Gatorskin is the only midlevel 27 but Nashbar no longer carries it-maybe Conti stopped making it?). If your wanting to convert to a 700 but are not sure the brakes will work just take the bike to an LBS and they can pop on 700 rim to see if it will work. Also they use to make a brake pad extension that was designed to extend the pad on short reach calipers so you wouldn't have to buy a whole new brake system...I don't know if that is still available though.