Hello
You can change the head unit. You should receive a card with the security code on it, or in the case of the one I bought it was written on a sticker on top of the unit.
The new head unit will just plug and play with no extra wiring needed, unless you went for the satnav version then you would have to run an antenna to it.
There are battery drain issues with the RCD510 on older model Golfs which need a new CAN gateway or something. There is further info in the forum some where!