I found this:
CD-RW: I have had very mixed results trying to record to CD-RWs. CD-R media has become so cheap, the added expense and unreliability of CD-RW makes them generally unnecessary. cdrecord can erase a CD-RW using the "blank=fast" option to erase the old content. Read the man page to learn more about the other options available to erase a CD-RW.
cdrecord -v -force blank=fast dev=0,0,0
Haven't tested it myself, I'm currently working on "the other system" ;-)