M.B.
Frankly, he is mostly reacting to the change.....
It is different, and he isn't used to it, so he isn't happy, thus he perceives her as "mean".....
Toddlers say a person is "mean" for lots of reasons..... maybe she is making him put toys away, and he doesn't want to.... maybe she isn't reading his favorite book... and he sees that as "mean" ..... maybe she isn't giving him the snack he wants...... you get the point?
He will have to learn to adapt to many changes in life.... it isn't always going to go "his way" ......
And, just as you saw at the beginning of the year, these adjustments may take a few months..... give it time.....