700c rims are approx. 8mm smaller in diameter than 27" rims, or more importantly they have a 4mm shorter radius, which is the distance between the center of the hub and the outer edge of the rim. Some of the older road brakes will reach, some won't. If you're already maxed out at the adjustment slot on the brakes with the 27" rims, those brakes aren't going to work with 700c rims. On an older bike I recently changed from 27" to 700c, I had to use BMX brakes on the rear to get the necessary reach. The old front brake worked, but barely. The BMX brakes I used are of the same basic design as a dual-pivot road brake, they're just a much larger version. I was able to pick them up at a local BMX shop for little to nothing, and they work just fine. I believe Harris Cyclery sells some BMX brakes they recommend for extremely long reach applications on older bikes..........The reason I went to the 700c rims is that I was looking for an economical, pre-built wheelset with a modern freehub that would accept an eight or nine speed cassette, and also I wanted to have a good selection of tires to choose from. So my advice would be if you're not upgrading to a modern drivetrain and you're happy with the 27" tires you can get, there would be no reason to change. If you're upgrading to an eight speed cassette or more, or you want the selection of tires available in 700c, then the change would be worth it.