sub-form to update main form field and retain old info

hello,

I have a form named installations that tracks equipment installed on utility poles by site name. e.g. serial number 123456 installed at site name B234

the serial number is a lookup to an equipment table.

I have a sub-form / table called service that tracks the number of visits to a utility pole  associated to the site name.

service has only a few options. 

audit, troubleshooting, repair request and remove and replace.

when choosing remove and replace, i would like the sub form to show the currently  installed serial number and provide a lookup to the equipment table for a new serial number.

when a new serial number is entered, I'd like to have the old serial number remain in the sub-from / table and have the newly entered serial number populate the installation form/ table with the new serial number...

can this be done?

thx

-N

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Nic M
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