Well, changelings transformation basically is said transmutation version of disguise self spell but whit no magic aura attached. They do have plain physical 'real' form wich is male or female. But changleing male can turn out pregnant while bearing female form and he cant turn to opposite gender during pregnancy.<br/><br/>Also changelings raised among humans (simply said changeling who disguised as husband to screw the wife or changeling passer mother replacing her child to pick real human child because fear of discovery). So most cases parents who have changeling children (and they dont kick them out or kill in anger) are not much loved because there is ALWAYS treachery involved why both pure blooded humans (apparently) get changeling child.<br/><br/>But somecases changeling child can disguise his nature quite some time, some cases whole his life, that he is changeling from his human parents if they are smart. Anycase changeling haves all physical traits of assumed form but no statical changes (nor gainign new movement methods like flight, swimming etc). Also disguise self is limited to humanoid forms whit limits of Disguise Self spell.<br/><br/>Anycase in Races of Eberron there is alot fascinating information about changeling's life :).