Kona Gallagher from TV Squad says my thoughts a little more eloquently:
"We've already briefly met Pam's mom back in season two, when she came to visit Pam at work (back when she and Roy were still engaged). At that time, she was played by Shannon Cochran. While TV shows recast roles all of the time (especially small, one-shot ones), it seems as though would be better about this. One of the things that makes the show so great is its attention to detail, and the fact that they don't underestimate their audience by just assuming they've forgotten plot points from seasons past."
Yes! Attention to detail- exactly. I would expect something like this from Days of Our Lives, but that is only because they do this all the time. The Brady character changes all of the time!