Please work with TAC as suggested by Herman Robers.
You may be able to run 6.6 and 6.7 clusters in parallel for some/needed time with the help of TAC instead of going through the below action.
"We have 2 Subscritions IDs and the Cluster with 2 nodes, each one with 5K Access.
What if we upgrade just the Subscriber license to 6.7 and let the Publisher active in 6.6? What will happen to cluster with 6.6?"
You will migrate the license keys to 6.7(NL) based on the subscription ID. TAC will be able to verify the subscription IDs and answers your queries.