An SSL reissue is means that your existing certificate is replaced by a new one. A reissued SSL certificate is only valid till the actual expiration date of the original certificate.
You need to request for a reissue in any one of the following situations:
- You have changed your web server/hosting provider
- You have lost the private key for the certificate
- You feel that your private key is compromised
- You have changed your contact information and you need to update it on your certificate