That's like saying the States can enact something that is contrary to the American Constitution. The Church established Church discipline with regard to praying. The decision was made by an Ecumenical Council and is binding to all particular Churches, who, individually, cannot change it on their own (hence the problem with inserting the Filioque as well).
Thus what the Church as a whole decides (through an Ecumenical Council), the Church as a whole can change (through an Ecumenical Council). To the best of my knowledge, no Ecumenical Council changed Canon XX of the First Ecumenical Council.
*No. Disciplinary Canons are not Dogmatic Canons. And I don't know of any general rule that disciplinary Canons are intended to remain in force in perpetuity unless specifically voted out of existence. It is true that subsequent Coun`cils specifically said that some Canons must remain in force but Councils would become excessively pedantic and legalistic were every Ecumenical Council required to re-examine, debate, and vote upon all previous disciplinary Canons.
That is not the purpose of an Ecumenical Council