Enabling Things iPhone legacy sync messes up recurring tasks and areas - can this be avoided?
Prompt response via email from Julia at Cultured Code to my support request using the feedback form answers the question:


  When you revert to Things 1 mode, the data from Things 2 cannot be
  converted to the older database format. There should be a note warning
  about this when converting to Things 1 mode; if not, I apologize for
  the confusion this caused.
  
  Basically, if you want to have the to-dos from Things 2 in Things 1,
  you have to re-enter them manually. Since this downgrade path is one
  that only very few customers chose to take, we didn’t spend time on
  implementing a downgrade path for the data.