Automated copying of Finder comments
This AppleScript code works for me using the latest version of macOS Mojave.

set originalFile to (path to desktop as text) &amp; "Original.flv"
set convertedFile to (path to desktop as text) &amp; "Converted.m4v"

set appendTextToComment to " This File Has Been Converted"

tell application "Finder"
    set originalComment to comment of alias originalFile
    set comment of alias convertedFile to originalComment &amp; appendTextToComment
end tell


Obviously you need to change the file path variables to the location of your files and the appended text value to whatever you want