ColdLeaf2015: Thank you for that very informative (and terse) post! I might add for those who might be slightly confused: you were laying out a Plan A course of action (1,2,3a,4a) and a Plan B (1,2,3b,4b,5b,6b).
One of the questions I have is the importance of initializing the CAN gateway device. It was not clear to me from their video if this needed to be done a) before disconnecting the old battery pack or whether it could be done b) after the old battery pack was disconnected. (I must really like your a,b usage!). If a), then it makes me wonder if there might be an initialization problem in the future if their gateway device failed (or needed its firmware to be updated) and had to be replaced with a another (or newer firmware) one.
A second question I have is related to my noticing that as my 24kWh battery pack ages its ability to accept regen power becomes less and less. I wonder how well the gateway device sellers are able to test for that with a limited set of (perhaps relatively new) battery packs. Perhaps they would be willing to use volunteer Leaf owners to use their EXISTING battery packs together with the gateway device to test it now, especially if it were easy to remove the device to restore original battery pack functionality if a problem arose.