But the same article provided the answer to your question anyway. Here's an excerpt:
"Removing powered-on stack members causes the switch stack to divide (partition) into two or more switch stacks, each with the same configuration. If you want the switch stacks to remain separate, change the IP address or addresses of the newly created switch stacks."
So based on the above logic, you should not power down the switches and just remove the stack cable between switch 2 & 3. That should effectively divide the stack into two stacks. Then you can just change the IP address of one stack immediately afterwards.
You have to think about this carefully , specially if the 3750 is doing any routing . If you separate the stack and it is routing then you have to have different ip addresses on each stack . Not sure what your reasoning is with this is but I would carefully review what you have now and what would happen if you break them apart .
Seperate the 2 switches from the other stack entirely. Modify the configuration as necessary to fill the new role. Create a trunk port to uplink your new 6-switch stack into.
The 6 switch stack will have one node assume the master role and use the previous configuration. Modify the configuration on the new master to have a new IP, no layer 3 routing info, etc. You basically are going to need to modify the previous config to support only the layer 2 configuration. Set up a trunk port to trunk to the layer 3 switches. Remove the old switches from the stack list (from 'show switch'). Set the default gateway to the layer 3 switch(es). Save the config.
Connect the two stacks on their trunk ports. Everything should be good to go.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...