if you still have Zbrush 3.1 you can export out your highest and lowest subDs as objs and then import them into 3.1 and generate the normals using Zmapper again. Just import lvl 1, subD to the same level as your highest, then import the highest. Then generate your nomrals. TADA. Kind of a pain, but at least you can get your normals.
make sure you save a morphtarget before you import the highest too.
Anyway, hope this helps.