The Sapphire decoders are not replacements for the very basic Hornby budget decoder, but are higher spec models.
I assume both the basic decoder and the Sapphires will be available in the range.
To answer the question about what Sapphires can do that the basic decoder cannot, is quite difficult to say without Hornby releasing the full specification, however it is known that the power rating will be higher (1 amp continuous) and it will be RailCom equipped. Sapphire decoders will almost certainly have a much fuller range of CV adjustment available too.
I've heard that certain journalists/authors may have been beta testing examples before they are released for general sale. If this is the case, then maybe Hornby are trying to avoid a similar debacle to that which accompanied the release of their first basic decoder, the awful R8215.
Of course the R8215 was modified twice before Hornby decided to scrap it in favour of the re-worked R8249.
For those who are interested in the Sapphire's fuel/coal and water load features, it may be worth pointing out that these are exclusively RailCom features and require the user to have a RailCom enabled DCC system and suitable feedback facilities in order to use them.