There are two option
1) You can either remove the hierarchy for the records that are involved in the subscription movement. Once the records are merged, you can re-establish the parent-child relationship. (Remove the parent at the child level and ensure there are no unpaid invoices while you are unlinking and relinking the subscriptions). Click here to know more.
2) You can delete and recreate the subscription. If you are concerned about losing the invoices, you can import them using bulk operation. However, please ensure you have taken export of all the invoices and the subscription before deleting it.
It is recommended that you test both of these options on your sandbox first to see it in action so that you can be aware of the flow. This will also help you to understand how the changes affect the records.