I am not sure why this changed from ArchiCAD 7 to ArchiCAD 8 but shouldn't the threshold of the door remain at the same level when changing a doors height and have the head height vary? For example, when you change a 6'-8" door to a 7'-0" door the threshold changes to -4". You then have to change a second parameter of the door to get it in the correct position. This is a complete waste of time and to me totally counter intuitive.
An argument can be made that windows should behave this way but for doors it's kind of ridiculous.