Odtnhj

I'm trying to implement something where I need to change values in the registry where what I'm removing and what I'm adding is only decided at run time.
my test code:

CString test_string, change_from, change_to;
test_string.Format(_T("Communications Port (COM1)"));
change_from.Format(_T("COM1"));
change_to.Format(_T("COM2"));
test_string.Replace(change_from, change_to);
MessageBox(test_string, _T("Test"), MB_OK);

works as expected.

but when test_string and change_from are populated from the registry it does not.

CRegKey serenum_key;
serenum_key.Open(HKEY_LOCAL_MACHINE, _T("System\CurrentControlSet\services\Serenum\Enum"), KEY_READ);
CRegKey sysenum_key;
sysenum_key.Open(HKEY_LOCAL_MACHINE, _T("System\CurrentControlSet\Enum"), KEY_READ);
DWORD count = 0;
serenum_key.QueryDWORDValue(_T("Count"), count);

CString keyname, keypath, friendlyname, portname;
const ULONG max_len = 256;
ULONG len;
keypath.Preallocate(max_len);
friendlyname.Preallocate(max_len);
portname.Preallocate(max_len);
for (DWORD i = 0; i < count; i++) 
 keyname.Format(_T("%d"), i);
 len = max_len;
 serenum_key.QueryStringValue(keyname, keypath.GetBuffer(), &len);
 CRegKey dev_key;
 dev_key.Open(sysenum_key, keypath.GetBuffer(), KEY_READ);
 len = max_len;
 dev_key.QueryStringValue(_T("FriendlyName"), friendlyname.GetBuffer(), &len);
 CRegKey devparams_key;
 devparams_key.Open(dev_key, _T("Device Parameters"), KEY_READ);
 len = max_len;
 devparams_key.QueryStringValue(_T("PortName"), portname.GetBuffer(), &len);
 ATLTRACE(friendlyname);
 ATLTRACE(portname);
 CString newname;
 newname.Format(_T("COM%d"), 50);
 ATLTRACE(newname);
 friendlyname.Replace(portname, newname);
 ATLTRACE(friendlyname);

 devparams_key.Close();
 dev_key.Close();

sysenum_key.Close();
serenum_key.Close();
}

the traces I'm getting show what I expect to see coming from the registry key, and the new name, but the friendly name remains unchanged:

Communications Port (COM1)
COM1
COM50
Communications Port (COM1)

why does Replace work in the first instance but not with the registry key values?
for note I'm working with visual studio community 2017, platform toolset v141, Unicode, x64

The documentation on CString::GetBuffer explains, how to use this class member:

If you use the pointer returned by GetBuffer to change the string contents, you must call ReleaseBuffer before you use any other CSimpleStringT member methods.

If you don't, the internal state of the CString object is not valid. The GetBuffer/ReleaseBuffer family of class members are useful, when you need to attach non-CString string data to CString instances. The former temporarily disable class invariants, where the latter re-establishes them, so they always need to be called in tandem.