I have walked this road and it wasn't pleasant or as simple as the documentation suggests. It took approximately 3-4 MX 250 RMA's, waiting on new firmware and bug fixes and about 2-3 years before we were able to successfully swap MX100 with MX250.
I think it was in firmware MX15.x that it was mostly sorted and we were able to replace mx100 with mx250 but yeah for us it was painful process.
Two quick questions regarding Quick Swap, Method 1 please:
1) Under the section: Remove the Old MX from the Current Network...
There appear to be an omission in that it says 'To remove an MX from an existing network, follow the steps below.' and then has no steps below, only a warning note regarding unique client identifier, (not an issue in my use case). I am assuming the missing steps are just the 'Remove appliance from network...' button under Monitor->Appliance Status. Correct?
2) I am assuming the step after completing Add the Replacement MX to the Same Network
and previous to physically connection the new replacement MX: Adding a Second MX to Form a Warm Spare Configuration should be marked 'Optional', as in our case we are not adding a pair of MXs? Or, is it in fact necessary to add the new, single replacement MX250 as a Warm Spare in order to get the configuration? I would assume that this step is not applicable, and that rather, after completing Add the Replacement MX to the Same Network, the new MX is available for monitoring and configuration and will configure upon physical WAN connection. Correct?