“Old” consents prior to the entry into force of the GDPR (25 May 2018) do not automatically become invalid. Consent may continue to be used as a legal basis for processing, provided that the nature of the consent already given complies with the conditions of the GDPR (Recital 171, third sentence of the GDPR).
If you are not sure whether your “old” consent meets the requirements, obtain new DSGVO-compliant consents. The Data Protection Officer will be happy to advise you.