It seems that Navionics haven't made things quite as clear as they could, but thanks to everyone who has offered advice.
There is now a "Navionics+" chart option, that sits between the Gold and Platinum charts, but this is only known as a Navionics+ if buying it outright. If you're upgrading from a silver/gold chart it's called the "Navionics Update"!!
You need to buy the update card, then insert into your PC with Navionics Chart Installer running. You'll be prompted to insert your old card into a PC to prove you have it, and then replace the update card back into the PC for the new chart to go onto. After the process you'll have a Navionics+ chart on the new card and your old silver/gold chart on your old card as backup.
And thanks to Daniel, who is currently trying to look after me with the update.
There's weather out there - must be time to RIB!
(Or dive, or ref rugby, or.......)