Populating A Reference Field When Parent Record Is Made After The Child Record

So, we've added a reference field to a child record that has already been submitted and we don't want to have to go back and manually fill in those reference fields for all the other child records that have been submitted. For example, let's say Appointments is the parent table, and Clients is the child. People have already submitted their Clients records, but we've recently added a reference field to the Client table and we want to autopopulate the corresponding Appointment record into that reference field. Is their any method or formula we could use to achieve this?

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Anaya Steward
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