Right now the only way to change a cars transmission is to look up the gear ratios for the transmission you want to swap after doing the full transmission upgrade. This is OKAY, but is a bit of an issue for say the 2016 Camaro where the manual 6-speed is modeled and going to the 8-speed, as even if I set the gear ratios for the first six gears there’s no way to add the final two.
I think having either a transmission conversion that you can purchase in the conversion section of performance, or just in the tuning tab once you have the highest transmission upgrade allow adding/subtracting the number of gears.