@BTibetanMastiff
When I was in the similar siutation we tried the decommission in batches (3 to 4 nodes at a time). CM/Hadoop will internelly take care of the replication (for the deleted data from decommission node), so no need to worry about the replication part.
So below are my recommendations
1. Decommission the nodes by batches
2. CM -> HDFS -> Web UI -> Datanods -> Decommissioning -> Make sure there is no under replicated blocks
3. CM -> All Hosts -> Commission state(left side menu) -> Decommissioned -> Make sure your host is listed
4. If you have time, wait for 1 or 2 days or more then delete the host from CM