Forum:Swords of Concealing Light question

Hello!

I was dueling someone today and activated Swords of Concealing Light to send all of their face-up monsters to face-down defense position. When I ended my turn, and he entered his main phase, he flipped the monsters from face-down defense position to face-up defense position. Is this a legal move? I was under the impression that the effect of Swords of Concealing Light made it impossible to change the positions of monsters already under the effect, although I'm aware that the opponent can still summon new monsters in face-up attack position. Any confirmation about this?