It’s not about converting the car.

I have a 2009 Chevy with an automatic transmission. I’m order to convert it to electric, the ECU would have to be replaced so the car knows when to shift to a higher gear without a combustion engine.

Because of environmental reasons, ECUs are pretty tightly controlled by the government. I don’t know if any company even exists that can sell an aftermarket ECU. There’s plenty that can hack or reprogram ECUs, but even that is becoming increasingly regulated and legally questionable.