Be sure to work with CivicRec support staff to get completely switched over.
Upgrading CivicRec from 2.0 to 3.0, Authorize.net changes:
- Log into authorize.net
- Go to "Account" at the top-right
- Go to "Response/Receipt URLs"
- Click "Add URL"
- Enter https://secure.CivicRec.com/hostedPayment/returnWebhookTransaction/2
- Click "Submit"