How can files be moved (not copied) from a directory on a FTP
server into another directory (on the same server) ?

In version 3.4.0 and later you can use FTPCommand statement with the SITE command.
In previous versions you can either use Telnet if you've got Telnet server running on your primary FTP host or install and run 24x7 Remote Agent on the FTP server computer.

: How can files be moved (not copied) from a directory on a FTP
: server into another directory (on the same server) ?