I’m on the same boat. Bought a 16-zone Rachio 2 two years ago, never used the “extra 8” zones. As a matter of fact, all I’m using are 6 zones of the 16 . . . .
Having said that, and considering I also work in the computer industry, I think this answer (“it gets difficult to map 16 zones to 8 zones”) is not very useful, or even correct. It is correct, I think, that it is IMPOSSIBLE (not “very hard”) to map existing 9 zones or more to 8 zones. But if the “source controller” is 16-zone, and only 8 or less are in use . . . what is the challenge here ?
At least, if not 100% automated from “16 (with less than 9 in use) to 8”, there should be an option to a guided migration, or at the absolute very least, to be able to “clone” a zone from the 16-controller to the 8-controller . . .