Recertification:
Recertification is the process of renewing one's certification. Recertification implies that you are building upon your knowledge, growing as a professional and increasing your experience daily. Recertification is the process where the candidate who already holds the said certificate, retakes the same at the end of the certification validity period.
Upgrade:
Upgradation is the process of replacing an existing certification with a better, improved and more relevant certification.