Several of my alts did change and learn new skills over time. I had an angry kid alter that was very 'one-sided' at first... ie: she was only ever angry, and always presented the same way. Over a long period of time this alter developed a therapeutic relationship with T and learned many new skills and coping techniques, and eventually learned to experience a full range of emotions. I don't exactly know how or why that happened, but she became one of our most loved and most trusted alters who could pretty much cope and deal with anything.
What I do remember about our work with her is that at first she was ONLY angry, and she was only ever out fleetingly - as soon as another emotion was elicited she left.
I say give it a go - helping anxious alts to learn ways to feel calmer is part of the process of healing. (It might take a while, though!)