terse

@echo off
set "file=a.txt"
set "str=*,,3000-0"
(for /f "usebackq delims=" %%a in ("%file%") do (
     set "line=%%a"
      setlocal enabledelayedexpansion
      if "!line!" == "!line:@=!" (echo,!line!) else (echo,!str!)
      endlocal
))>b.txt
pause
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