Following up on my own post...looks like 12-27 cassettes are only available in 9-speed. I need 10-speed. Ultegra CS-6700 comes in an 11-28 or 12-30. Looking on ebay, appears I'll have to pay at least $80 (probably more, with shop markup) to get that, excluding labor; and then whatever they'd charge to redo the brakes. And I would probably need a longer chain and possible a new derailleur as well, if my bike doesn't use the long cage. One thing leads to another, I guess!

I'll have to wait to decide when/if to spend the money -- was hoping not to have to put a lot more money into bikes for awhile as I've just bought two bikes in the past half year, plus some gear, tires, etc.