Hi @rainer_zufall, good question!
Upgrading to 1.8 does not include the storage upgrade for FP1 users. For now this upgrade can only be done from 1.6 - so I strongly advise doing the storage upgrade on 1.6 and then upgrading to 1.8.
Sorry for the hassle, but there was no way to unify the storage on your phone without wiping it, as such it can not be done in a “normal update”.