Windows – REG ADD REG_SZ where Value contains embedded double quotes (redux)

This is driving me bananas but it must be something very simple. I am trying to modify an ImagePath Value (REG_SZ) for a Service in a Batch script using REG ADD, where the Value Data contains embedded "double quotes". But I keep getting an "Invalid Syntax" error. This is the Value I am trying to add:

Key  : HKLM\SYSTEM\CurrentControlSet\Services\myservice
Value: ImagePath REG_SZ
Data : "c:\program files\mydir\old.exe" -helloworld

I am trying to change the ImagePath to:

Data: "c:\program files\mydir\new.exe" -helloworld

However I am getting a Syntax Error in the REG ADD command. This is the stripped down script:

@echo off
setlocal ENABLEDELAYEDEXPANSION ENABLEEXTENSIONS
for /f "tokens=2*" %%A in ('REG.EXE QUERY "HKLM\SYSTEM\CurrentControlSet\Services\myservice" /v ImagePath') DO set IPATHOLD=%%B
SET IPATHOLD=%IPATHOLD:\0= %
echo OLDPATH^=%IPATHOLD%
set "OLDEXE=old.exe"
set "NEWEXE=new.exe"
for /f "delims=" %%A in ("%IPATHOLD%") do (
    set "string=%%A"
    set "IPATHNEW=!string:%OLDEXE%=%NEWEXE%!"
)
echo NEWPATH^=%IPATHNEW%
@echo on
@pause
if !IPATHNEW! NEQ !IPATHOLD! (
    @echo ready to change
    @REM next line results in Syntax Error
    REG ADD "HKLM\SYSTEM\CurrentControlSet\Services\myservice" /v ImagePath /t REG_SZ /d "%IPATHNEW%" /f
)

I have tried enclosing %IPATHNEW% in "doublequotes", 'singlequotes', [brackets] and \backslashes\ but I keep getting the syntax error.

Any idea what I am doing wrong?

(Note – this is a replacement of this question:
REG ADD REG_SZ where Value contains embedded double quotes)