In case you imported subscriptions via the API as described here.
Update the execution date via the user interface
You might want to update the execution date of the imported subscription in order to synchronize this with the old system. This can be done either via the UI if you open the scheduled subscription and update the Date:
Update the execution date via the API
Alternatively you can also do this via the API by using the Shopify Recurring Order Service with the Update Endpoint