As the topic states.

I was wondering if I can remove my current ECU and have my other OEM ECU programmed on the same key.
This way I can swap them out on the dyno.

It would make sense that I can but I wasnt sure if they "imobilize" the old ECU

 

ECU immobi code is part of the hardware parts of the ECU. You cannot change it.

You will need a key for each ecu.

 

But you can replace ECUs so they must disable the old ECU then according to your answer.
My current ECU (04)is not the factory one and ueses my original key (02).

 

They do not "disable" the old ECU. The key is programmed to the ECU. Not the other way around.

 

As of 2010, The same key can be registered to more than 1 PCM. It still needs to be done at a dealer. You would install the new PCM, register the keys, reinstall the original. As long as the 1st PCM registered keys are not deleted the same key would start in both. It is entirely possible that this is no longer true with current Honda HDS software.