The challenge is to get some person (servant, doctor, Earl, visitor...) from 1100-1700 to pass a 3 month old infant girl to an English countess and get her to think it's hers and continue breastfeeding it. She can't be mentally disabled, in other words she needs to be as smart as a noble would be in those days.
This is harder than modern "hospital switched the babies" thing. The reason is that a lot of newborns with the same natural hair color look similar (and a lot of them are born covered in hair for extra visual obstruction of features). The Lanugo of newborns is obviously gone by 3 months.
Now where can one find an infant that looks like the recently born girl? And wouldn't the replacement behave incorrectly compared to the child it's replacing? Man, this is hard