Just for the record, you don't need to reformat anything to downgrade from 3.2 to 3.1 (or any other >=2.0 version).
Just use the 3.1 bootcd and choose to "upgrade". This will "upgrade" your 3.2 install to 3.1...

Of couse this won't work for a G5 which is only supported in 3.2+.