I looked in the Form (Payment Entry) to see if I could use a SQL statement to select the data (dates), but that doesn't appear possible.
I would like to investigate two approaches; first in a (this) Form and second as a query that might make the changes in a batch process. Joe Copying data from one record in a table to another on a form is possible with a macro created for that specific purpose.
With Zapier, you can configure Airtable forms to edit and update records in any of your Airtable bases.
This is especially useful for cases where you would like someone to be able to edit part of a table without having access to the rest of the base that the table is in.
Last Renewal Date and Member Thru Date are updated in Payment Entry Form when dues are paid.
The requirement is that a Family Member, which looks just like a Primary Member, except for a P or F, carries the same date information as the Primary.
A word of caution: this is a fairly complex Zapier integration.
The next step is to connect your Airtable account to Zapier (or to select an already connected Airtable account).
In this Zap, the trigger will be a new record appearing in our duplicated table, which will happen when a record is submitted through the form. Zapier will take the project name in the new record to find a matching record in the original Projects table (the table that Zapier will be editing).
Last, if Zapier finds a matching record, it will reference the field values from the record submitted by form to update any fields in the original table.
Knowing a bit more now about what is wanted presented new problems. I would have placed these family records in a different table linked back to the primary account records.
Not only would this eliminate the presented problem but seemingly a variety of other duplicated information such as address, city, state multiple indicators for family and possibly more.