I dont think you can change the commanders. AT least not the ones which are sold.
Well bulletins make an impact, if the summary of 3 increase the survivability of your main infantry.
It's a damned shame really, because the whole commander system is worse in every way to the old doctrine system. Not only is it worse, but it was clearly done just so they could make it another thing to milk money out of you for and now they can't really "overhaul" them because they sold something specific.
I assert that they are worse because A) They are only 5 abilities and not 6. B) it is a linear tree and does not branch and C) there are far too many that overlap and are poorly thought out. And D) the callins they provide are unhealthy for the game.