I had my 9Z for years. I had my 14MZ for 7 years. I'll try to get at least that out of my 18MZ.
Like you, I plan to keep the 18MZ a long time.
I had the 14 MZ for 8 years, and just upgraded.
It was easy to cop[y and transfer all the models fro the 14 to the 18.
I had planned to keep one 14MZ as a back up., but I have to change the plans: at Joe Nall, I tried to go the other way, by copying a new model from the 18MZ, and transferred it to the 14MZ....result was bad!!!
The copying from the SD card went fine, but instead of loading the model, the 14MZ froze on a blank screen and no button worked anymore.
I went to the Futaba booth, and no one was able to unlock it. Frank took it back with him to get it restored to normal state.
Bottom line, the models are transferable only in one direction, from 14MZ to 18MZ.
I am now waiting for the release of the 18SZ which is supposed to be fully compatible as a back up system.