Editing a file without overwrite permission

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http://unix.stackexchange.com – Is it possible to set permissions for file to keep it editable but without permission to overwrite? I mean possibility to edit the file with text editor, but denying any attempt to replace the file by overwriting. The user is root. I know that it is not the common structure of unix file permission, but I am looking for a tricky approach. I have a package of files, as some of them have been edited/customized from the original version. When updating the package with a new version, I want to replaced any untouched file, but protect the customized files (not to miss edits). Currently, I must do (HowTos)