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I want to copy just the card from one customer record to another without having to ask the customer to enter it again. How do I do this in Chargebee? 
I don’t want to merge all the subscriptions and invoices but just the customer’s card details to another

Summary

If you have two customer entities having their invoices paid by common payment details, you can look at setting up a parent-child relationship using our Account Hierarchy feature. However, if the hierarchical setup is not what you’re after but are only looking to copy the card details from one customer to another (with the customer’s consent), you can follow the steps below. 

Choose the Bulk operation “Create using Permanent Token” from Settings > Import & Export Data > choose Bulk operations 

You should provide the target customer ID, payment_source[reference_id] -This can be found from the existing customer’s payment record (see image below), gateway account ID (Settings > Configure Chargebee > Payment Gateways and open the gateway that the card already exists in. Stripe in this case) 

Sample upload: 

You can always refer to our API guide for this operation to know more about the fields and format. 

Warning:
If you wish to delete the Card from the first parent record, use only the Delete Payment Method from Chargebee operation and not “Remove Payment Method” from the card section directly. Using the latter will delete the details from your gateway too. Locally deleting the Payment method from Chargebee is available under the List of bulk operations > Payment Methods 

Note: 
Moving the payment record from one customer to another will result in subsequent invoices being charged on this customer’s card. This should be done with the customer’s awareness and consent that they are accepting charges for another record in Chargebee too.

Related articles and Documentation

Create using Permanent Token API 

Bulk operation in Chargebee