Change the SCART sockets? No chance!

Given that you have 4 devices and only 2 sockets you are going to have to use a SCART switch, unless you enjoy groping around behind the TV every time!
The configuration to use will depend on whether or not your DVD player can be set for RGB output. If it can, us the switch to select between the DVD & Dreamcast on AV1 using RGB, and daisychain the Digibox and VCR into AV2 using Composite Video. If not, leave the DVD on AV2 using S-Video, and use the switch to select between the Dreamcast and DSS/VCR on AV1. You will have to select between AV1 and AV1RGB on the TV as well.
SCART switches are available from Tandy or Maplin, but make sure you get one suitable for RGB, not all are. I know Maplin do a suitable two way switch.