As an aid to writing gender-shifted characters, let's look at some of the factors that would affect the behavior of such a character, particularly how their behavior changes. Most of these actually apply to any transformation, really.
Gross Physical Changes: "Gross" as in "large." In addition to the innie/outie shift, and the possible addition/subtraction of breasts, there's height change, altered center of gravity, hair growth patterns, pelvis angle and the like. This is likely to be the most noticable and immediate source of behavior changes. What clothes will fit the character, how high they can reach, seat restraint settings, how they walk, a constellation of large and small adjustments that have to be made. Depending on the kind of transformation, these changes may be difficult to adjust to, or be done unconsciously.
Subtle Physical Changes: Changed hormone cycles will affect moods, especially before the character becomes used to the new circumstances. This may result in emotions being less controlled, and nasty mood swings. Also, there's at least some biological component to gender identity and sexual preference. If those "brain switches" have been flipped, there's likely to be some conflict between what the body thinks it should be doing, and what the character remembers as appropriate attitudes.
Social Pressure: Other people will react differently to the character who's shifted genders, in both obvious and subtle ways. (And this is likely to be amplified if they know about the gendershift.) Their altered reactions will in turn influence the character's behavior. (Deciding not to change to conform to others' expectations is in itself a behavior change.) Indeed, in societies with very strong gender roles, the character may be forced to change their behavior in some ways on penalty of physical punishment.
Psychological Factors: The character's own attitudes towards gender roles, sexual preference, conformity and the like will also influence how their behavior changes due to a gender shift. Someone whose self-image depends heavily on virile masculinity is going to have a rougher time dealing with being turned into a girl than, say, someone whose self-image is primarily based on being a good, charitable person.
Mental Changes: Actually, a bit of a cheat--if the transformer changes the victim's memories or attitudes by force, this will naturally change the gendershifted person's behavior. "You now think that childbirth is something you'd really like to do."