Modifying part of registry key

I need to change one character in a registry key on over 600 XP SP3 machines.  I could spend the next week or two doing it manually, but I'd rather use some other way.  The key is [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon]
"DefaultDomainName There are ten characters: the first six refer to the machine's location, the remaining four form a unique identifier.  I can write a reg file to change the whole key, but I need to change the second character and only the second character.  Can anyone help?