Hi Dar

I'm sure TE'ers with personal experience will pitch in here... I've not done this yet on my mixte but I just did it with my Surly Cross Check. Since a triple crankset is wider than a double, you'll also need to replace the bottom bracket (for a longer one) and the front derailleur (for a longer one). Does your bike shop have the right ones to fit your mixte's frame, or can they get them?

Best of luck!