I feel like the change gives an opportunity for new avenues in the story and life lessons about maturity, responsibility, and, probably, jealousy. It as if people forget about the part where Twi asks Princess Celestia if she will still be her student and is answered with basically 'yes, but it will be different'. I feel like that is a pretty direct way of saying, "don't worry everyone, everything isn't going to completely change". I guess I really don't mind the change, but if Twi gets reverted back...well, I wouldn't mind that either.
The only things I keep asking myself are: 1) How are episodes going to end/will they still end with lessons? 2) How is Spike going to fit in with Twi's new role?