When your old transmission had problem, your transmission control unit (ETC or TCU) went into limp mode (2nd and reverse only). As long as the codes are in the ETC, it shuts off power to the solenoids inside the transmission.

When you replaced with a different transmission, the codes in the ETC are still there so there is still no power to the replaced transmission.

An OBD II scanner cannot clear the codes in ETC. It seems the aamco folks are a bit challenged in this situation.

That does not mean the transmission they put does not have problem or your ETC isn't bad but you need to have the codes cleared before anything can happen.