Question: after software upgrade are the alfresco doc node ID preserved ?

Answer is: yes
According to Alfresco representative, document nodeid are not modified during a software upgrade. NodeID are stored in the database and so they keep the same value (as upgrade require a DB copy).

Also on a cluster, both cluster member will work with the same nodeID.
On a cluster, the only elements which might not have the same ID are the lucene indexes UUID (but this is not a problem because each cluster member has its own index copy).