The system will determine whether the provided AccountId is for a credit card or an ACH object, and will make the default account change as applicable.

See Understanding Accounts for more details.

Current Credit Card Set for CustomerId 255939:
{
    "CreditCardNumber": "************1111",
    "ExpirationDate": "12/2019",
    "Issuer": "Visa",
    "BillingZipCode": "80433",
    "CustomerId": 255939,
    "IsDefault": true,
    "Id": 395560,
    "LastModified": "2013-11-07T22:11:08Z",
    "CreatedOn": "2013-10-11T17:41:36Z"
}, {
    "CreditCardNumber": "************5454",
    "ExpirationDate": "08/2018",
    "Issuer": "Master",
    "BillingZipCode": "80202",
    "CustomerId": 255939,
    "IsDefault": false,
    "Id": 396036,
    "LastModified": "2013-11-07T22:11:08Z",
    "CreatedOn": "2013-10-15T18:33:25Z"
}

Request: PUT   /v4/customer/255939/396036

Updated Credit Card Set for CustomerId 255939
{
    "CreditCardNumber": "************5454",
    "ExpirationDate": "08/2018",
    "Issuer": "Master",
    "BillingZipCode": "80202",
    "CustomerId": 255939,
    "IsDefault": true,
    "Id": 396036,
    "LastModified": "2013-11-07T22:11:08Z",
    "CreatedOn": "2013-10-15T18:33:25Z"
}
{
    "CreditCardNumber": "************1111",
    "ExpirationDate": "12/2019",
    "Issuer": "Visa",
    "BillingZipCode": "80433",
    "CustomerId": 255939,
    "IsDefault": false,
    "Id": 395560,
    "LastModified": "2013-11-07T22:11:08Z",
    "CreatedOn": "2013-10-11T17:41:36Z"
},