I changed my server software

During the validity of your certificate, you may have to change the machine, or more simply the server software. For example, switching from Windows Server 2008 R2 to Windows Server 2016, or switching from IIS to Apache.

In order to prevent any trouble when the time of renewing your certificate will arrive, you can request a reissuance and get a certificate especially made to work on your new configuration. For most of server certificates, this operation is free and is way to keep your certificate (and the associated information) up-to-date.

This operation is not always compulsory from a technical point of view: it is sometimes possible to export and then import the certificate and its private key. The disadvantage is that you will not be able to renew your certificate for the new software, and will then have to buy a new certificate (renewal is sometimes cheaper and keeps the anniversary date).

Further information about reissuance, see here