newswan

@echo off
setlocal enableDelayedExpansion


for /f "delims=" %%i in ('dir /b /a-d /s "*.docx"') do (
    call :f1 %%i
)
goto :END

:f1
set filename=%~n1
:loop
if exist "!filename!.doc" (
    set /a filename+=1
    goto :loop
)
ren "%1" "!filename!.doc"
goto :eof

:END
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newswan

@echo off
setlocal enableDelayedExpansion


for /f "delims=" %%i in ('dir /b /s "*.docx"') do (
    call :f1 "%%i"
)
goto :END

:f1
set filename=%~n1
:loop
if exist "%~dp1!filename!.doc" (
    set /a filename+=1
    goto :loop
)
ren %1 %filename%.doc
goto :eof

:END
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子目录，没注意你那里有 /s 参数