The word "definitively" does NOT make this a definition, because the entire document is written in past and present tense, and says NOTHING about what the Church can or cannot do in the future.  Therefore, it is "definitive" about the past and the present, but cannot possibly be definitive about the future, because it says NOTHING about the future.  

Can you find, in the entire letter, a verb conjugated in future tense?  If so, please let me know.  My understanding is that "Ordinatio Sacerdotalis" makes CCC 1577 "definitive" as long as we *believe* that the choice of the 12 male apostles, a choice made under the OLD LAW, also applies under the NEW LAW.  But this is a ludicrous and fundamentalist exegesis. 

The power of the keys was given to the Church to OPEN the doors of the reign of God, not to close them.  "This catechism will thus contain both the new and the old (cf. Matthew 13:52), because the faith is always the same yet the source of ever new light." (Fidei Depositum, 2).