Once you have booked your exam, you cannot reschedule to a different mode for the same attempt. However, you can choose the exam mode again, while scheduling the next attempt.
For example: if you have scheduled your first attempt at the test center then you can’t reschedule that exam to an online proctored exam at home or vice-versa. However, you can again choose your exam mode to test at a Test Centre or Online Proctored at Home when you schedule your Retake 1 or Retake 2.