This is perfect for my use case, thank you! The .fodt file is 150K, vs 117K for the .odt file. But after changing a word less than 20 of the 1983 lines in the file have changed.

  $ ls -hl todo.fodt
  -rw-rw-r-- 1 dotancohen dotancohen 150K Apr  7 16:41 todo.fodt
  $ ls -hl todo.odt
  -rwxrwxr-x 1 dotancohen dotancohen 117K Apr  6 12:01 todo.odt
  $ wc -l todo.fodt
  1983 todo.fodt
  $ cp todo.fodt todo-edit-one-line.fodt
  $ libreoffice todo-edit-one-line.fodt
  $ diff todo.fodt todo-2.fodt | grep "+ " | wc -l
  19