"SITE PSWD old new" is a non-standard extension of the FTP protocol and it is not implemented by all FTP servers. In fact, I guess that most FTP servers don't support it (and some only support it for a subset of authentication methods).
Some FTP servers might use a different command than
SITE PSWD. For example, WS_FTP uses
"SITE CPWD new" command to change the password.
Since this functionality is server-specific, our advice is to consult the documentation of your FTP server for more information.