Can Link monsters be switched to defense?
From the Official Ruling, page 15, first paragraph:


  Link Monsters have no DEF and cannot ever be in Defense Position. 
  They can’t be changed to Defense Position by a card effect. They 
  can’t even be flipped into face-down Defense Position.


We can see that Link monsters can't ever be in Defense position, not face-up or face-down. This also excludes changes from card effects like Book of Moon.

More info on this answer and question.