How can I increase the amount an existing monthly sustainer is giving?
This won't work, because CiviCRM doesn't communicate with Authorize.Net regarding changes to recurring billing.  Only token-based payment processors (e.g. iATS) do.

While I can't answer specifically for Authorize.Net, in PayPal you can change the billing amount by no more than 20% every 6 months without entering new card info.  You do this from the PayPal UI.