Before_rename or after_rename - Merge With Existing and custom field


When the doc is renamed from the form
and “Merge with Existing” box is check

How can we define what fields to inherit ?
For example I have a custom doctype
And only want to copy my custom status field from the:
renamed doc to merge with the old doc

Package Label: xyz, has status: In Customs
Package Label: 123, has status: Unknown

Both are the same package, but after we verify the label physically we can merge then

Open: Package Label: xyz and “Merge with existing” Package Label 123

and its merge the status from 123, into xyz. So the final doc has status: Unknown

Maybe im doing it wrong, this can be modified in: before_rename or after_rename ?