Since I’ve not pulled the mast on our C380, I can’t definitively speak to the conduit in the mast. Yet, I’m pretty confident that you can use the old radar cable to pull the new digital cable through the existing conduit. From the diagrams in the manual, the digital cables look to be about the same diameter or smaller. They should be smaller since coax has been replaced by digital signal wires, along the more efficient digital scanner drawing less current.
The Raymarine digital radar manual provides extension cable part numbers. These are likely the appropriate part numbers, but check to be sure.
2.5 m (8.2 ft) extension cable A92141
5 m (16.4 ft) extension cable A55080
10 m (32.8 ft) extension cable A55081
Historically it wasn’t a good idea to splice coax radar cables, but now that the cables are digital, it’s probably not as critical. If you feel