I only noticed a permanent change in one mare, and she was a QH mare. She had always been rather cantankerous, but became SO mellow after her first foal. Had I known that would happen, I would have bred her earlier, LOL. It would have made my life a lot easier! Lady had her first foal at 7 years old in 1997, and she's still much easier to deal with all these years later.
We do have one mini mare that gets very friendly and loving late in her pregnancies, but then she reverts back to normal after the foal is born. She's never been unfriendly, she just wants a lot of loving the later the pregnancy gets, LOL.