700C and 29" tires fit the same rim diameter (some constraints on width, though, since 29ers are usually kind of fat). 700C and 27" are not interchangable. To convert to 700C, you'd need new wheels and the brakes would have to be adjusted down 4mm. If you have that much downward adjustment range on your brakes, you may be able to upgrade to 700C, but it really isn't necessary. Continental's Sport 1000 is a very nice affordable tire available in 27x1 1/4 size. Your LBS can order them if they don't stock it or you can get them at a number of places on line.