converting a one-way relationship into a join-table
I have a field in one of my talbes that I originally related to another table via a single field and one-way relationship. Later on I realized that I needed a many-to-many relationship and set up a join table. The result is a redundancy; older records are linked via the one-to-many relationship, and new records via the join table.
Is there a way to automatically add an entry to the join table for all of my previous records so that I can simply delete the now outdated one-to-many field relationship?