It's Not Called the Presider's Creed
Is it proper for a celebrant to ad lib the Nicene Creed during Mass?
No, the presider at Mass may not deviate from the Creed in the text of the Roman Missal. The presider may lead the congregation in either the Nicene Creed or Apostles Creed but may not ad lib his own version.
22. 3. Therefore no other person, even if he be a priest, may add, remove, or change anything in the liturgy on his own authority.
26. Liturgical services are not private functions, but are celebrations of the Church, which is the "sacrament of unity," namely, the holy people united and ordered under their bishops