According to Paragraph 13 Section 5 of the Authorize.Net® Payment Gateway Merchant Service Agreement:

Authorize.Net may amend this Agreement at any time upon written or electronic notice to You of not less than ten (10) days prior to the effective date of such amendment; provided that the addition or change of service fees, will become effective upon at least thirty (30) days' notice. If You do not agree to such amendments, Your sole remedy is to immediately terminate this Agreement upon written notice to Authorize.Net.

To view the updated agreement, please view http://www.authorize.net/files/Authorize.Net_Service_Agreement.pdf