Going from a 27" rim (ISO 630) to a 700c rim (ISO 622) means a nominal radius change of 4 mm for the bead seat. Your rims may vary in how they translate that into the braking surface location, but it could well be that your existing brake calipers have enough adjustment to make up the difference. I have, in cases where it was real close, filed the slots to get a little more. But I'm not sure that I'd recommend that generally speaking. Also worth noting is that the braking surface on old 27" is at a slight angle, which old brake pads had molded into them to match. Any modern aluminum wheel has vertical braking surfaces, so you'll need to replace the pads with something that can be adjusted square to the rim in order to have a full contact.