parent to child table snapshot of field based upon text selection and update of parent

hello,

I'm trying to archive a field from a parent table to a child table. then update the parent table.

I have a parent table. "device_location" (key is record ID) and a child table of "service" related to parent by Numeric (reference) proxy field of "device location name" 

the parent table also contains a lookup to a master table of device serial numbers.

A device serial number is placed at a location in the parent device_location table. occasionally  a device needs to be removed and replaced in the service table.

when creating a service record I will select a service action from a drop down list. the service is called R&R (remove and replace).

I would like  to have the child table retain (snapshot?) the old serial number from the parent table (based upon R&R selection) and then have a field for the "new serial" to be entered and have that update the parent table.

not sure if this can be done?
and this may not be the most clear way to ask?

thank you

-N

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Nic M
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