On the 722.6. According to an aftermarket trans repair manual, changing internal gearing of the trans will result in an ratio error. ( MB used 2 ratio sets between 4 / small 6 and large 6 / V8 / V12 , Chrysler has 3 more from what I recall. ) In this case the different ratio is interpreted as clutch slippage and the trans will drop into limp mode.

There may or may not be an issue with changing the rear gear. The only info a trans may get in relation to rear gear is a speed signal from the RF ABS sensor ( this supposedly runs the speedo ) and maybe a rear wheel speed sensor / pinion speed sensor.

A really quick way to test is to pull the trans computer from the rear you want to use and install in your current car.

And, if there is an issue, building a CAN filter would solve the problem. A CAN filter looks for a specific information designator and value on the CAN bus and replaces it with a different value. In your case you need to make the trans / engine computer think the car has the old rear ratio. Look at some Euro sites that offer odometer filters for the theory. ( I have not tried a rear swap )