I have a Tiagra equipped bike that I use in the Pyrenees, and Want to change the rear ratios. It is a Tiagra double, with a 12-25 at the moment, and I am looking to change to a 12-27. I wouldn't have thought this is going to be a problem but those are usually famous last words...
I am spending three months there in the summer, but will take an ultegra 10 triple 12-25 with me when I go. My Trek 1200 I leave there all the time, so when I pop over to see the in-laws I have a ride available. In time I probably will change it to a compact - my commuter bike is a compact 36-50 with 12-25 and it is surprising the difference it makes.
For the moment a new cassette is cheap and easy though.