Coincidently, we are doing this on my wife's 3X9 (got an 11-42T Advent 9 speed cassette and the derailleur and shifter are on the way). We are still trying to decide on the crankset to get, so thanks also for including that in your post.

(I bought the cassette first to see if I could make it work with the current 9-speed XT derailleur. Close, but no cigar. In retrospect, maybe I should have made this a 1X10 with AdventX 11-48T cassette).

Also converting the kid's 1X12 to an Advent X, after trashing two Eagle and two Shimano derailleurs.