Lots of 700c to 650B conversions have been done, but 27" to 700c takes it a bit further, as the difference in wheel radius is even greater.

Technically, no modification was necessary-- any frame that takes non-canti rim brakes can be a candidate for conversion, but you just want to make sure you have sufficient clearance for fat tires, and that the BB height is reasonably high, to avoid heel strike after conversion. I settled on this frame because initial measurements suggested that 42mm Hetres would fit, and the BB was really high (288mm-- typically BBs are in the 270-280mm height range). Converting can result in the bike being anywhere from 10-25mm lower. Mine got lowered by 13mm, bringing the BB down to 175mm.

But I really wanted cantilever brakes, so for me the only critical modification was repositioning the canti studs 23mm lower, to work with 650B wheels. All the other modifications were strictly cosmetic:

- lowered the rear brake bridge 10mm to fit a fender without needing a long L-bracket.
- drilled holes with reinforcement rings for internal wiring of the dynamo wiring
- added a third set of bottle cage mounts

While at it, I had the rear triangle re-spaced to 130 to fit a modern cassette hub.

If you have a frame builder near you, you may be able to have all this done for under a couple hundred dollars. I was fortunate in that I had one-stop shopping for all the work, including the powder coating. It was done by Geekhouse, here in Boston. Marty, the owner/founder, studied under Mike Flanigan of ANT, so he has expertise both in building steel bikes and in powder coating. He has his own powder coating business within Geekhouse (Sugar coat). The powder coat on this bike is far better than the PC on my other bikes.