Very very easy. As long as you stay within the physiological limits of any one existing creature type. Note that there's a chance for the individual to change mentally as well as physically, though with the chance of death by system shock, you obviously shouldn't genderbend too often. (but since you can cast raise dead, you might not need to worry about that so much.)
I'll also let you switch out cleric spells for druid spells if you want, while you're creating your character.