When changing Merchant Processors, Authorize.NET will provide a new API Transaction Key. In MC Trade 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.
Go to:
1. In the MC Trade Browser, select Admin -> Global Settings
2. Click Integrations from the left menu.
3. Find your existing gateway, click on the Actions menu on the right side of the screen, and then click "Edit" to pull up the settings for this gateway.
4. Click in the "Authorize.net API Login Key" and "Authorize.net API Transaction Key" boxes and update the existing information with the new login key and API Transaction Key.
4. Then 'Save' the changes at the bottom.