When changing Merchant Processors, Authorize.NET will provide a new API Transaction Key. In Atlas the existing gateway can be updated with the new API Transaction Key. You can generate the new API Transaction Key on Authorize.NET.
Do not delete the existing Authorize.NET gateway.
1. In Atlas select Admin -> Set Preferences -> Application Settings
2. Select the Credit Card Processing tab -> Select Edit/View Credit Card Gateways
3. Select your existing Gateway and then double-click in the cell below AuthNET API TranKey and remove the existing key and copy/paste the new one from Authorize.NET:
4. Then 'Save' the changes at the bottom.