Ok, I understand what are you asking. The changeling is a creature with all the types, included "Wall". The walls can't attack, that is your question.
"Recently" magic the gathering has changed. Don't exist the walls, i mean, they exist but the rule "The walls can't attack" doesn't exist. Actually all the walls are considered a creature with the "defensor" ability. Defensor ability doesn't let attack.
The changelings don't have the "defensor" ability. They can attack