I've found this can be a bit of a pain to do!
if at any stage you get stuck, you. an raise a support case via the ServiceNow training site here
example of how they helped me:
Thank you for contacting the Now Learning team.
Your accounts have been merged.
NowLearning account firstname.lastname@example.org has been de-activated and all the learning history has been transferred to NowLearning account email@example.com.
Webassessor account's ID has also been linked to this nowlearning account firstname.lastname@example.org
Happy Learning ahead!
I am moving this ticket to resolved state.
Your ticket will be resolved but remains open for the next five calendar days in case you are still experiencing an issue. Please respond to this case within the next five calendar days if we can be of further assistance.
Moving the certs yourself
Basically, in order to move the cert you can do this from the partner portal profile as per the following ServiceNow video
via my profile> admin> my certifications, you'll then spot the button 'search certification to map/transfer'