Is Mam slowly becoming a substitute for Ma'am as a lot of people don't know the difference between the two.
Ma'am is used in UK in the armed forces and the police when a junior addresses a female superior officer. It is also used to address Her Majesty the Queen.
Mam is used for "mum" in Yorkshire (see Alan Bennett's diaries, for example).
To answer the direct question: I have seen no evidence of Mam replacing Ma'am, nor, until seeing this Question, had I heard of such a possibility.