You can recertify at any time. Please note, however, that you must recertify prior to your certification termination date.
Please visit the recertification page associated with the certification program you are interested in to find more information on recertification.
SRC: https://www.nokia.com/networks/training/src/recertification/
Nuage Networks: https://www.nuagenetworks.net/learning/certification/recertification/
ONC: https://www.nokia.com/networks/training/onc/recertification/