Originally Posted by chrisesposito
You can do that, but if you do, that may not be all you need to do. From what I remember, most road rear derailleurs have a limit on the difference between smallest and largest cogs that they can handle, and so an 11-27 is about at the limit. Putting a mtn bike rear derailleur on there ought to work just fine.
Your local shop will be able to advise if the rear derailler will need changing, and that is a possibility. The other thing is a longer chain aswell, so yet another expense. Taking it that you want to retain the 52 on the front, then this is your best option.