The cab shell is the same between a 9650sts and 9750sts in the same model year. The differences will be in the originally installed options. For example...field office or no field office. The other differences will be in the armrest and cornerpost. All of the buttons should be on the armrest, you might have to uncover buttons and change addresses (and possible do some calibrations...depending on option again) to make features/options match up.
As far as the cornerpost goes, 9650sts has to match up with 9650sts and 9750sts has to match up with 9750sts because of its communication with the engine controller. You might have to get a new cornerpost and have a dealer install it to make sure that all of the addresses are aligned and have the correct inputs. Just a word of caution . . . don't think you can do the cornerpost yourself. It may work for awhile (a grace period), but it may quit working after that grace period is over and hours haven't been programmed. Then you'll definitely need a dealer...because you'll need a new one or have to ask them to try to send it back to the factory to be reset.
Good luck,
Chad