the mods converting the automatic transmission to a reverse valve body are not trivial. not only is the valve body modified, but a few other areas of the transmission are also altered. If you were already skilled at rebuilding the 727 transmission, I suspect you would find reversing the changes, and locating the missing and unaltered parts a real challenge and costly.
if it were me, I would seek out another used 727 transmission, and begin preparing that one for use in the car.
I don't believe the case, itself, is altered by these modifications. if the numbers on the first case are "numbers matching" to your car, and your car is a valuable optioned car, then you're faced with moving the guts from the other transmission back into your numbers-matching case. Also not an easy task, but easier than undoing the reverse valve body mods. especially if the transmissions are from similar applications and/or part numbers.
even if you have deep pockets, and insist on reversing the changes, it may be a challenge to find a shop that has the experience to make the repairs.
if you need to source parts, and get answers and specific technical support, I can recommend A&A Transmission. They have always had my back, and provided the parts and guidance I needed to get the job done.
Home - A&A Transmissions
That said, teaching one how and what to do, over the phone, is probably beyond the scope of the services they offer.
If you DO want to move the guts from one case to the other, go find the part number on the oil pan lip (PK-number) of your current transmission to help guide you in selecting the second transmission. The PK-number will also help you determine if the trans you have now is the original numbers matching.
For example, if your transmission is from a 1973 car, and you pick up a 1967 transmission from a swap meet, you're gonna have some challenges in a few areas. but if by some miracle, you find another transmission with the same PK-number, it may become trivial moving the guts into the original case.
