Fictional characters are free to put through changes.
There's no rule that says they must remain locked into an unchanging mold in order to be credible. Often times changes are needed in order to keep a character fresh and maintain interest.
As for justifying any change with a "properly justified good story", judging such a work is entirely subjective. As much as some will complain about what Aaron is doing with Jen, others will say they love or like the choices he's making.