Hopefully, this should not happen as a funeral home would have thought who in their contacts of Celebrants would be best suited to the family. However, if this does occur, then again, you simply tell the funeral home you have changed you mind. The same would work for myself. If I visited yourself and you decided that I was not what you had expected or found myself to be not what you had wanted after all, then you notify the funeral home and tell them you want someone else.
I have had this exact thing myself where a client decided they wanted a vicar even though I had been to visit them.