Of course Deena changes, but she changes in the way that everyone else changes. The grows older, more mature, and grows tired of show business.
Not to say that no one cares how she changes, but her change isn't the important one.
Effie's change is deliberate, definite, and most importantly... *musicalized*. Effie's AIATY is a towering aria to how she will not move, which is soon there after contrasted to her Act II scene and song, aptly titled, "I am Changing".
Yes, Deena is young naiive and she grows up to be a competent performer and eventually able to move on from Curtis, the time spent on Deena is considerably less than Effie's, and she serves as a great juxtaposition to Effie.
But if Deena took the lead/main focus, Two of Kreiger and Russel's best and most powerful songs loose their power and poignancy.
And all for Beyonce?
I say it's not worth it.
Personally, I think I have too much bloom.