Remplacer un caractere se trouvant en position 81 par un autre

@echo off
Setlocal enableextensions enabledelayedexpansion

cd %USERPROFILE%\Desktop

:debut
cls
set source=C:\vini\test.txt
if not exist %source% goto :eof

set destination=fichier2.txt
set /a pos=1

for /f "tokens=*" %%A in (%source%) do (
set ligne=%%A
if !pos! EQU 1 (call :action) else (echo !ligne! >> %destination%)
set /a pos=!pos!+1
)

:: del /F %source%

notepad !destination!

Endlocal

exit

:action
set partie1=!ligne:~0,80!
set reste=!ligne:~81!
echo !partie1!X!reste! > %destination%
goto :eof
rem

@echo off
Setlocal enableextensions enabledelayedexpansion

:debut
cls
set source=C:\vini\test.txt
if not exist %source% goto :eof

set destination=C:\vini\fichier0123456.txt
set /a pos=1

for /f "tokens=*" %%A in (%source%) do (
set ligne=%%A
if !pos! EQU 1 (call :action) else (echo !ligne! >> %destination%)
set /a pos=!pos!+1
)

type !destination! > %source%
del /F !destination!

Endlocal
exit

:action
set partie1=!ligne:~0,80!
set reste=!ligne:~81!
echo !partie1!X!reste! > %destination%
goto :eof
rem

@echo off
Setlocal enableextensions enabledelayedexpansion

set source=C:\vini
if not exist %source% goto :eof

set historique=%source%\histo
if not exist %historique% mkdir %historique%

set temporaire="%USERPROFILE%\fichier0123456.txt"
if exist %temporaire% del /F %temporaire%

rem
rem
rem

if %source:~0,2% EQU %CD:~0,2% (cd %source% || goto :eof) else (cd /d %source% || goto :eof)
set /a nb=0

for /f "tokens=*" %%A in ('dir /b "*.txt"') do (
set texte=%%A
call :traiter
call :numeroter
type %temporaire% > %historique%\!fichier!
del /F %temporaire%
)

del /F "%source%\*.txt"
explorer /select,"%historique%"

Endlocal
exit

rem
rem
rem

:traiter
set /a pos=1
for /f "tokens=*" %%Z in (!texte!) do (
set ligne=%%Z
if !pos! EQU 1 (call :remplacer)
if !pos! GTR 1 (echo !ligne! >> %temporaire%)
set /a pos=!pos!+1
)
goto :eof

:remplacer
set partie1=!ligne:~0,80!
set reste=!ligne:~81!
echo !partie1!X!reste! > %temporaire%
goto :eof

:numeroter
set /a nb=!nb!+1
set fichier=fichier!nb!.txt
if exist %historique%\!fichier! (call :numeroter)
goto :eof
rem