I would get a pro to do it. First and foremost.
So that would mean the local B&O shop.
You would need to locate the right wiring for the plug and then solder it back on.
Any B&O shop could provide that at least.
If the cable is not long enough you would need B&O to open the bottom part anyway to change the whole cable.
Its not a big repair.
You can try Beotech to find the wiring diagram.
http://www.beoworld.org/beotech/index.htm
I don't know if the AV9000 service manual is even uploaded but if you upgrade 2 silver level you got access to it.