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.

Problem:
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.

Question:
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?

Anyone?