directory shell context menu item to rename file extensions

I have used the following registry mod successfully in win XP to add a right click option on a folder to rename m4a (AAC files) to m4b (bookmarked AAC files).

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Directory\shell\m4b.m4a]
@="Rename m4a to m4b extensions"

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Directory\shell\m4b.m4a\command]
@="cmd.exe /c \"rename *.m4a *.m4b\""

Unfortunately, it is not working in Windows 7 32 or 64 bit. If I open a command prompt in the folder, the rename command works as advertised, but for some reason it won't execute properly from the context menu based on the registry key.

Update:

Running the reg command without the terminate option (/c) showed a command prompt at the %windir%\system32 folder. So the command was executing, just in the wrong folder. Looking at the open-command-window-here reg setting, I saw that it uses the selected directory with the pushd "%V".
Apparently XP runs cmd.exe from the selected directory by default.

So I chained that command with the rename and viola it's working!

Here's the finished reg entry:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Directory\shell\m4b.m4a]
@="Rename m4a to m4b extensions"

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Directory\shell\m4b.m4a\command]
@="cmd.exe /s /c pushd \"%V\" && rename *.m4a *.m4b"

MilesAhead had posted to this forum with the correct diagnostic before I had a chance to post the fix. Thanks for the help. It would have got me to the same point.