Nice bike.
In my opinion, don't bother switching from 38x700s to 32x700s, i was following the same train of thought you were (skinnier wheels = less rolling resistance) and i switched from 37x700 continental countrysides to 32x700 specialized boroughs on my commuter but I've found that pedaling effort is about equal, and if it is any better I can't really notice it. It's probably not worth tossing aside your sets of 38's. I say ride em out and replace when worn.
As for the 29er part of your question. As aforementioned 29er rims use 622mm rims but are generally thicker. I tried mounting my 29er tires on my commuter (kona dr.dew) for fun but there was not enough clearance between my chainstays to handle the 29x2.3" tires. The rims on my kona are also quite a bit skinnier than my 29er as well (19mm vs 27mm). I don't know how thick your rims are but You may have some trouble mounting some of the wider 29er tires. So keep that in mind. Hope that helps.