Is it possible to change the 'Key Usage' property of an existing ssl certificate of a web site? Our goal is to host a web service so that a third party can integrate with our solution, and their requirement is that our ssl certificate has 'Data Encipherment' in the 'Key usage' field. Currently our ssl for the site doesn't. Trying to see if it is as simple as somehow editing existing certificate without having to buy a new one.
Certificates are signed. The signature covers all the certificate contents, including the
Key Usage extension. You cannot change anything in the certificate contents, not the smallest bit, without invalidating the signature. So no, you cannot "change" a certificate. Certificates are immutable by construction.
Your certificate vendor might accept to give you, or sell you with a discount, a new certificate which fits your requirements.
It might depend a bit on the certificate vendor's policy, but usually changes in the certificate require you to buy a new one.