The date mix up

They shouldn't have had to change it at all and Emma should have decided to stand up to the wedding planner rather than pick that time to stand up to Liv. Of course, there would be no movie with the ensuing hijinks but it would have made more sense for her to have told the wedding planner that she already booked that date and the woman that came in afterwards would have needed to take the other date.
She could have used another opportunity to stand up to Liv and for that matter, her fiance (still before tying the knot with him). I don't think he nor Liv were jerks, just used to being in control and Emma let them be. She even thought her mother would be upset if she picked another dress but her mother told her she would have been fine with whatever she wore.
Of course the good thing about how it all went down, it did cause Liv to change.