Enterprise e-mail. How to handle certificates of resigned users?

Hi all,

Couldn’t find relevant info on the knowledge base after a quick initial search.

We have purchased several corporate e-mail certificates to sign e-mails for key persons within the company.
Now, we have a situation where 2 of these persons have left the company.

How to handle this? Can we revoke the certificate and reissue it to another employee (typically the new hire to refill the available position)?
Do I go to the E-KPI manager to reissue the certificate?