Who changed it?
Based on what authority?
If we have a church writing something for 100 years can it alter the Commandments?
If it is a tradition to worship idols, does that change the Commandments?
Constantine did formalize something which had been going on previously, but sanctioning something that is contrary to scripture is not an amendment to the scripture.
Good thing St. Constantine did not sanction something that is contrary to Scripture.
The authority comes from Christ who gave the Apostles the authority to loose and bind.