You really not giving us enough information to work with. Topology map would've been useful.
That being said, please make sure that:
Two switches (Nortel and Cisco) are interconnected by a trunk with all the VLANs present in Nortel also present on Cisco and allowed on that trunk.
Depending on particulars of your implementation you may want to switch CCP to a broadcast from multicast, if not already done.
Move standby member to Cisco and attempt the failover. I suggest doing it in a maintenance window approved by the company.
Once the unit connected to Cisco is active and you have verified the health state of the cluster, move the remaining unit from Nortel to Cisco.
These are suggestions, not instructions, so use them at your own risk.