A customer's subscription interval cannot be changed to a different frequency; the option is unavailable in the dropdown menu. How can this be corrected?
The available subscription frequencies are determined by the subscription group settings. To change a customer's subscription frequency to another value, the desired frequency must first be added to the customer's subscription group.
More details on subscription frequencies can be found here - https://support.boldcommerce.com/hc/en-us/articles/29371478117780-Subscription-Group-Settings#subscription-frequency.