Q: When a church that has (presumably) already been dedicated becomes the Cathedral of a diocese, is there a special ritual (not a re-dedication)? Was there any special ritual prescribed prior to Vatican II?
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A: I know of no special liturgical ceremony that upgrades a church to a cathedral. There will probably be some Vatican document, and the nuncio may be involved in reading it. But no rededication ritual takes place. Even before the Council, I see nothing in the pontifical for this.