Should be a straight off swap, finished off with a gearing adjustment.
Although, as others have mentioned, a 28/32 is a fairly low gear to start with. But if you've confirmed that you really need lower, go for it.
Admittedly, a supposedly mtb crank will open up lower gears than a 34T cassette would.
Losing top end shouldn't be much of a concern. I run a 42 big on my commuter and am entirely happy to coast those precious moments when I'm spinning out.
Or you could look into some of this stuff:http://www.abundantadventures.com/quads.html
I've gotten really fond of "road" (=tightly spaced) cassettes, and something like a quad, or a ridiculously tiny granny gear, would make it possible to get both a decent overall range, and tight spacing.