Hello Uli,
I'm unclear on your question. Are you asking is it possible to revert from Advanced to Essential license? If so, yes it is possible using the following commands:
- config t
- svs switch edition essential
As we're only talking about the VSM, the VEM modules will still forward traffic and should be unaffected by this change.
Please review the following documentation: http://www.cisco.com/c/en/us/products/collateral/switches/nexus-1000v-switch-vmware-vsphere/qa_c67-717571.html
-AJ