setlocal enabledelayedexpansion
set /a first_index = 14
set /a last_index = first_index
:LOOP
if exist %last_index%.json (
set /a last_index += 1
goto LOOP
)
set /a last_index -= 1
for /l %%i in (!last_index!, -1, %first_index%) do (
set /a j = %%i + 1
ren %%i.json !j!.json
)
endlocal