In one of the last episodes of the series (before it became Mayberry RFD), the plot revolved around Emmett's 25th wedding anniversary, with Emmett trying to figure out what to give his wife for the occaision (#245, "Emmett's Anniversary"). Andy suggests that Emmett and his wife come over for dinner. He then mentions that Aunt Bea is "out of town" (one of the standard excuses used to explain away a missing cast member), but says that his temporary housekeeper, "Mrs. Pendleton", will talk to Emmett's wife during dinner to find out what she wants as an anniversary gift.

It seemed obvious to me that it was originally intended for Frances Bavier to have the lines given to the actress who played Mrs. Pendleton. It also seems to me that there may have been a possibility of Frances Bavier not returning to the show at all (which explains why they felt another housekeeper was needed), otherwise, they could have written this one episode around her.

Was Frances Bavier ill? Was there some sort of contract dispute, or other disagreement? Does anyone know the reason for this last-minute substitution?

dont really know jimbo, i think towards the end of the series francis bavier wasent to happy on the show so it was prolly over a dispute of something.