*snip*
One option: play ZG under the PF rules until "enough" of it has come out in 5e, and then switch over. You could just do this at an arbitrary time with no in-game justification, or...
[sblock]The climax of book 9 involves breaking the laws of reality itself. Starting in adventure 10, magic and physics itself is reforming. If you needed an in-game justification to change rules systems, you couldn't have asked for a better excuse. FWIW, this happens when you hit (PF) level 17, so it might be a bit late if you're trying to avoid the teen-levels altogether.[/sblock] *snip*
I am not sure that would work. My best guess is, at the current rate, a table will play through the adventure faster than the 5e version is released regardless of whether they are playing in 5e or Pathfinder.