Preview a plan change

{  "currentPlanId": "string",  "targetPlanId": "string",  "interval": "monthly",  "amountDueNow": -9007199254740991,  "currency": "str",  "nextRenewalAmount": -9007199254740991,  "effectiveAt": "2019-08-24T14:15:22Z"}

{  "defined": true,  "code": "UNAUTHORIZED",  "status": 401,  "message": "Authentication required or credentials invalid.",  "data": null}

{  "defined": true,  "code": "FORBIDDEN",  "status": 403,  "message": "You do not have permission to perform this action.",  "data": null}

{  "defined": true,  "code": "NOT_FOUND",  "status": 404,  "message": "The requested resource does not exist.",  "data": {    "resource": "string"  }}

{  "defined": true,  "code": "CONFLICT",  "status": 409,  "message": "The request conflicts with the current state.",  "data": {    "reason": "string"  }}

{  "defined": true,  "code": "VALIDATION",  "status": 422,  "message": "The input failed domain validation.",  "data": {    "issues": [      {        "path": "string",        "message": "string"      }    ]  }}

{  "defined": true,  "code": "RATE_LIMITED",  "status": 429,  "message": "Rate limit exceeded; retry later.",  "data": {    "retryAfterSeconds": -9007199254740991  }}