My transition was without a hitch. There is a cropping issue in 3.2 which needs an immediate fix.
I have stuck with Snow Leopard while waiting for the tick-birds to debug the first release of Lion.
Someone has posted on Apple's Aperture Discussion Forum that Libraries can be "demoted" from 3.2 to 3.1 simply by using the "Repair Library" option on launch of 3.1x. This makes sense -- but I haven't had the need to try it