While testing the encryption at rest with hashicorp vault.

If there is data files on mongod folder, It returns error while starting.

Following is the message.

{"t":\{"$date":"2021-09-28T01:37:36.858+00:00"}

,"s":"E",  "c":"STORAGE",  "id":24248,   "ctx":"initandlisten","msg":"Unable to retrieve key","attr":{"keyId":".system","error":

{"code":2,"codeName":"BadValue","errmsg":"There are existing data files, but no valid keystore could be located."}

}}

Is there any enhancement the applying the encryption at rest into existing environment?

(I know rolling applying from secondary is the solution of that, I'm just wondering other solution on it like partial encryption)