Do you need clearance from ATC to switch to a different approach at non-towered airport?
Quoted from the Instrument Procedures Handbook, page 4-6:


  APPROACH CLEARANCE
  
  The approach clearance provides guidance to a position
  from where you can execute the approach, and it also
  clears you to fly that approach. If only one approach pro-
  cedure exists, or if ATC authorizes you to execute the
  approach procedure of your choice, the clearance may
  be worded as simply as “… cleared for approach.” If
  ATC wants to restrict you to a specific approach, the
  controller names the approach in the clearance—for
  example, “…cleared ILS Runway 35 Right approach.”
  
  When the landing will be made on a runway that is not
  aligned with the approach being flown, the controller
  may issue a circling approach clearance, such as
  “…cleared for VOR Runway 17 approach, circle to land
  Runway 23.”


So if they name the approach, you are restricted to that approach.

I searched the rest of the book for items specifically related to non-towered operations, and found nothing that contradicted that.