The majority of the internals are the same, there are some changes but nothing mechanically that would preclude using it. The difference is due to how the fluid operates. When a transmission is cold to achieve a certain shift speed, TCC operation and clutch apply time the amount of pressure required to make those things happen is different throughout a fluids operating temperature. Thicker fluid requires more pressure vs a thinner fluid. So in this instance using a thinner fluid in an older transmission that is going to be asking for more pressure than the fluid needs to make a shift when cold would cause harsh and misapplied shifts. If using a thicker fluid in a transmission that requires a thin fluid it often causes a "flared" shift because the flow rates are different. If Mazda were to back service with the fluid it is very likely that older TCM programming would exhibit incorrect behavior with the different fluid. That would also require new programming to roll out for the older units. Its just not worth an OEMs time to do with some rare exceptions like GM with the 8L90. The 24 CX-5 2.5t uses a new TCM program vs the 23 for example.
The other concern you can run into with the wrong fluid being used is its coefficient of friction. During a shift you are contending with "dynamic" vs "static" coefficient of friction. Dynamic is how the friction changes during a shift vs static being once the clutch set is fully applied. Shudder concerns can happen with the dynamic coefficient of friction is incorrect for the friction material that is being used. There have been a few friction material updates throughout the Skys life up to this point. Sometimes Mazda just changes things to be Mazda.
