Double to triple?
It’s expensive and not easy to convert double to triple, as you need not only the crankset but also triple-type STI controls, plus front and rear mechs. That’s virtually a whole new transmission, apart from chain and cassette.
Get just the new-in-2013 105 long cage mech (RD-5701-GS), however, and you can increase the rear sprocket to 32, which will give a very slightly (0.8%) lower gear with a 34 ring than the 30 by 28 on your triple bike. Of course, you’ll also need the new cassette (an 11-32 out of the MTB stable) and a longer chain, but it’s still simpler and cheaper; STIs are the big cost, plus all the faff with cables and re-taping handlebars.
Hear this from a long-time exponent of triples: doubles work better! So if you can get all the gears you want that way (and now we have ten at the back, you possibly can), stick at two rings.
This was first published in the October / November 2014 edition of Cycle magazine.