Since the iA is such a new car, you would be the first (to my knowledge) to try it! So have at it and be sure to report back!
I have thought about it but from what I have seen, I ESTIMATE you would have to change the headlights and bumper cover at least. Not sure about the actual metal bumper; since the Mazda2 sedan didnt have to meet US crash safety standards, the plastic bumper cover might not fit over the (US approved) iA's bumper. I would think that if there is a Mazda stamp (or part number) on the actual metal bumper, then I assume it was designed to US standards and made to fit both bumper covers. A lot of assumptions here.
Headlight wiring CLIPS might be differents (since the headlights are Toyota/Scion parts), but I'm pretty confident the wiring harness would be the same (you may just have to source Mazda 2 wiring clips).
I'm pretty sure that you would not have to change the front quarter panels. The headlights are VERY close in shape at that point (and i wouldn't be surprised if they were the same). The shape difference seems to only affect the bumper.
Here is a site to visually compare.
I initially wanted to do the same (because I really wanted a Mazda 2 and not a Scion), but given I would have to import the parts, I figure I will likely just deal with the Scion front end.
Just curious: are you just swapping over the obviously different exterior pieces, or going to do a FULL swap? Technically (besides the front), the tail lights, gauge cluster, entertainment center, interior fabrics, and glass are all unique to the iA.... thats a lot to swap!