The obligations under the Money Laundering Act apply to both old and new clients, so the identities of old clients must be verified, if it has not been done earlier. Identity can be verified using a document such as a passport, or by using strong e-identification using banking codes, for example.
The information provider must retain the name of the client used in the verification of identity, the document number or other identification data, and the party granting the document or a copy of the document, or if the identity of the client is verified electronically, the information provider must retain the identification data of the procedure or source that was used.
If your old clients have already identified themselves electronically in such a way that the aforementioned identification information used in establishing identity has been saved and is in your possession, the verification of the identity of your clients already meets the requirements of the Money Laundering Act.