Originally Posted by Gonzo Bob
I would not be hard to change a 12-27 to a 12-30. Get a loose 30T HyperGlide cog. I don't think they make a 10-speed 30T but it can be 8- or 9-speed since the cog thickness isn't that different. Take the 16T loose cog and its spacer out of the 12-27 cassette. Put the 30T cog on the freehub body (with the 1mm spacer first, if required). Put on the spacer from the 16T. Put on the rest of the 12-27 cassette (without the 16T).
Edit: I just noticed that Sheldon has a page on this -
http://sheldonbrown.com/k7.html#custom
Edit2: I also just read that this method may not work for 10-speed because of the way the biggest cog fits onto the freehub. Looks like you are stuck with a 27T largest cog.
No, they do NOT make a 10-speed 30t. You make it sound so easy. Maybe I do need to be a bit more inclined to do more work on the bike myself but, so far, at least, the local bike shop has not been too put out by my acquiring ODD parts from 3rd party vendors. It is more their reaction to the unorthodox ideas and going beyond specs.
AND I just noticed your EDIT. I was just looking at that same site and was not sure what was or was not possible. I thought it might be but ..... That's why I'm asking.