How can I change the interval on all my existing subscriptions?

Here is how you can bulk edit all existing intervals in your subscriptions.

In order to do that, go to Seal Subscriptions → Automations → Automated interval changes → Create automated interval change. Then, below you can pick the option to apply the automated interval change Immediately on all existing subscriptions, and then you can set the delivery interval that you would like to change.

If you don't wish to change the delivery interval immediately, you can also pick the setting to apply the change only when a new subscription gets created or renewed.