To clarify the low level steps would be:
- Remove mx65 from original network
- Remove mx75 from cloned network
- Add mx75 to original network
For some additional context, the above steps roughly follow the method outlined in the Meraki doco, with some minor changes (essentially a hybrid of the 2 methods).
https://documentation.meraki.com/MX/Other_Topics/MX_Cold_Swap_Replacing_an_Existing_MX_with_a_Differ...
The overall logic is that the initial check-in, firmware upgrade and config pull take the most time (sometimes upwards of 30 mins). By pre-staging this (using the cloned network) it should speed up the overall process.
In the weeds a bit, but one other factor to think about is order of operations when moving the mx75 back to the original network. If the WAN IP is statically set and is different between the staging (cloned) network and the original one, you'll need to ensure it has a valid internet connection when moving it back to the original network, and/or manually configure the IP via local status page so that it can connect out and grab the updated config.