The Nicene Creed, the creed of the First Ecumenical Council, held at Nicaea, was modified at the Second Ecumenical Council, held at Constantinople (the section on belief in the Holy Spirit was expanded and the anathema at the end was dropped), thus the creed that we say in the Liturgy is more properly called the Nicene-Constantinopolitan Creed. The Second Ecumenical Council decreed, and the subsequent Ecumenical Councils confirmed, that the Nicene-Constantinopolitan Creed should not be altered in any way, shape, or form. Thus, it would be a clear sign of either heresy (Filioque) or of heinous marring of established ecclesiastical order and tradition to make or suggest any kind of change. In other words, messing with the Nicene-Constantinopolitan Creed is a red flag, making one\'s Orthodoxy questionable because the Nicene-Constantinopolitan Creed is the symbol of our Orthodox faith.
To clarify Namees point on Ecumenical Councils, these are NOT known only after the fact to be truly ecumenical in authority. Fr. John Romanides points this out in one of his essays. The Holy Fathers who participated in the Seven Ecumenical Councils (leaving aside 8 or 9) believed those councils to be ecumenical, at the time of their meeting. There was no waiting for general acceptance by the laity. True, there were other, heretical councils called and ecumenical status for them may have been claimed at the time, but this does not disprove the prevelant opinion, contemporary to the times of the Seven Ecumenical Councils, that they were ecumenical, that is, having authority for the whole Church.