Welcome to Beoworld.
You will need a Beolink converter 1611, which converts datalink to Masterlink. You have an AUDIO AUX LINK at your BM4500. Connect this plug with a DIN cable - 7 pin - to the BL converter and the ML socket to your BL2000.
May I ask what guitar you want to trade in? If it`s a 1959 Gibson Les Paul Standard or a white 1965 Fender Stratocaster it could be a bad deal....for you.
Hope this helps.
Stefan