As this is not RP/SE you simply use the RP DM Replacement Wizard to switch the Gen5 RPAs with the Gen4 RPAs. No need to spoof the WWNs. You then use the RP DM RPA Addition Wizard to add the Gen4 RPAs back in as RPAs 3 & 4. Both processes are NDU.
Hello,  We currently have 2 RPA clusters consisting of 3 GEN4 appliances. We will be upgrading from 3.5.SP2 to 4.x in a few months, remove fabric splitters, and then adding 2 GEN5 appliances to each cluster. In the 4.1 release notes it mentions that in a mixed RPA type (GEN4 & GEN5) cluster environment that it is recommended that the GEN5 appliances be set as RPA 1 and RPA 2 for improved system function and performance. I was hoping to get some clarification on that point.  I'm assuming after adding the new RPAs to the clusters as RPA 4 and 5 I would have to use Deployment Managers' System Modification option to set the new RPAs as RPA 1 and RPA 2. Is this correct?  Are there any other things that need to be considered prior to doing that?  Would this be disruptive to replicaton in any way since both cluster control RPAs are being switched to different RPAs in both clusters.   Thanks for your time.   
Changing RPA1/2 to different RPAs