Basically the farther or more severely you move away from the basic original design and original components of your bike, then higher the odds that you are going to run into something difficult. And I would put ''messing with wheel diameter'' as one of the most likely to run your head into a grinder. I have seen maybe 50 posts from folks starting down the path of moving up or down the diameter puzzle and it always result in a lot of puzzle solving and a lot of money spent to get somewhere near yet another next generation Frankenbike.

So basically my question is "why are you starting down that path"? What is the real motivation. Your frame already has canti studs so it already has the better potential for great brakes and the studs are ugly and/or hard to remove if you aren't using them for brakes. You already know that the 27'' wheels you have now fit your frame and the brakes, etc. If you rear hub is a freewheel hub and the new shiny cogset you just bought is for a shimano freehub that is a no-go, then you should consider that you could just put that cogset aside for your next bike and just go buy another freewheel to fit your existing hub for almost no money (assuming that the existing freewheel is broke or the wrong color or made insulting squeeks at your BFF?).

Above provided just to give you some thinking support before you get fully engaged with the uber geeks here in the Borg Bike Collective (sorry but BBC is taken) and spend thirty hours reading wildly differing opines on the obvious-to-a-blindman merits of 700C vs 27 inch vs 650B... when the real fact is that all of them are round and roll around pretty good. If you find that you want/need a specific type or size of tire and don't see them for sell to fit your rims, then post on that and guaranteed the Collective knows 13 places where they are for sale and on sale in the normal colors plus mauve and purple. [

Hope this helps...