Performing a rolling cluster conversion from NetWare 6.5 to OES is one of the easier ways to upgrade Cluster Services to OES and keep your cluster up and running during the process.
In this method, one server is converted to OES while the other servers in the cluster continue running NetWare 6.5. Then, as needed, other nodes can be converted to OES incrementally until all servers in the cluster have been converted. Although it is feasible to use a mixed NetWare and OES cluster temporarily as an upgrade strategy, Novell does not recommend it as a permanent production implementation.
Refer to Converting NetWare Clusters to OES Clusters
in the OES 2015 SP1: Novell Cluster Services NetWare to Linux Conversion Guide for instructions on performing a rolling cluster upgrade.