If this is a one time fix, just enter find mode, put a lone = in the clientID field and perform the find to find all records with an empty clientID field. This can also be scripted if you need to do this on a regular basis, but I think you can design your system so that such task need only be done once.
To avoid creating a new record unless ClientID has a value:
If [ not IsEmpty ( yourtable::ClientID )]
Thanks so much. Worked like a charm.