Option A: Cosmos Hub 2 and Cosmos Hub 3 should not be running simultaneously. It will be a mess if Cosmos Hub 2 still running when Cosmos Hub 3 is online. This is not an option.
Option B: Testnet is always available to test new software features. Unless there is new major software release, we don’t have to bother new testnet. I agree we need an upgrade simulation.
Option C: To simulate the Cosmos Hub upgrade, @bharvest is suggesting to fork Cosmos Hub 2, start a new network with another
chain-id. This is purely a simulation/practice of the upgrade process. If you would like to use your testing node to simulate this upgrade process, you can simply replace the validator key in your testing node to run this. A single validator running multiple validator nodes can be easily achieved. I think we already have enough keys to start this simulation. I don’t see why this is an unreal scenario. If this is smooth, I think we should make this as a best practice for future upgrade.