This will parse your %PATH% environment variable and convert each directory to its shortname equivalent and then piece it all back together:
@echo off
SET MyPath=%PATH%
echo %MyPath%
echo --
setlocal EnableDelayedExpansion
SET TempPath="%MyPath:;=";"%"
SET var=
FOR %%a IN (%TempPath%) DO (
IF exist %%~sa (
SET "var=!var!;%%~sa"
) ELSE (
echo %%a does not exist
)
)
echo --
echo !var:~1!
Take the output and update the PATH variable in environment variables.