One of my clients offered a free month added to a customer's ongoing subscription. As far as I can see in the ARB documentation here, I don't think there's a good way to do it. There is also no apparent way to do it when logged into the Authorize.Net platform.
The only good option I see to achieve this is to delete the subscription and create a new one. We do not store the user's full credit card information on our servers; I have no way to do it without the user's intervention, which isn't appropriate.
This seems like it would be a fairly common use case, so I was wondering if you had any insights in how to do it that I may be missing?
The best options for you would be to either refund the transaction for the month that is free, delete the subscription and create a new one manually, or use an existing third party web application or subscription manager which has the capability to automatically delete and re-add a subscription.