It seems the new AM Z is not showing up in my build list in my current save where I had already researched the tech that unlocked it.
Is it supposed to not retroactively unlock or something?

It is, I currently don't know how to use the migration, I hoped it would just update

if you don't mind problems with the achievements /c game.player.force.recipes['assembling-machine-x'].enabled=true will fix it in a savegame, meanwhile i'll find out how to migrate

Hooray its working. Thanks for the awesome mod :thumbsup: