A recognized historical document from the approximate time period of the switch would do just fine.
Do you find anything unreasonable about that requirement?
You are seeking a proclaimation by the Church? This would be difficult to do because most Illness and Calamity and Vocational Patron Saints weren't established by official decree, but by tradition. In tradition, they serve the same purpose as the pagan gods they replaced; the protect a particular group, or prevent a particular kind of illness. There is a patron saint of weavers and a goddess of weaving, patron saint of vintners and a god that protects vintners, saints for fishermen, gods for fishermen etc.