Is Plusnet saying that I now have to:
* learn how to backup the database and all the Wordpress files associated with the site
* move it myself to a different CGI / SQL server
In due course, you will be needing to move your site over (and as you say that will involve backing up and moving them yourself to the new server).
However, there is no rush associated with this. You have the option of starting the migration as soon (or not) as you like. The old platform is likely to be online for about 6 months more, and might well be longer if the plans take longer to implement (they often do!). Once the new platform is up and running in it's final state, there will be further communications that the time for moving is approaching. However by then there will be a wealth of experience available from the pioneers who went before you, so there should be extensive help and support available to make the move over as smooth as possible. Hopefully, there will be step by step guides available to help move the main CMS systems over (like Wordpress, Joomla etc), although these will depend on how helpful other customers are at sharing their experiences!
* pay more for the service
There is no cost associated with the move over. There is no monthly fee charged. The only situation that you might find yourself paying for the service is if your usage in a month is more than the allowances offered. The old allowances were in daily figures. The new allowances are monthly but less. Most people will find them to similar in real terms. The old limits were hard enforced caps, with auto archiving of the site when the limits were burst (I think that still applies to CCGI platform? Can anyone confirm?). For sites with irregular traffic patterns, they might get more traffic in a month under the new regime, since they can have much more on a given spike day without triggering a burst. For sites with a very level, consistent flow of visitors every day, they will get less traffic under the new scheme.
However, the limits are set as they are for the preview. By the time the platform is released those limits might have been expanded anyway. Even if not, then the new caps are more flexible anyway. Plusnet would work with you to get your site into line, if you were bursting the limits regularly, but the occasional peak isn't going to get you disconnected.
and, if it all goes to hell, I'm going to get no help or support ?
The support for the new platform (as with the old really) is mainly through the forums. This is because it is a value add service, rather than a core component. You won't be left high an dry though. I think by the time the move happens, there will be a lot of support articles already available, and questions will be able to be asked in here.
You can obviously, ask for a preview account as soon as you desire (although only ask for one if you will use it relatively quickly, as they might charge you to create a second one if you let the first one lapse through lack of use). You can keep your existing site working as it is on the CCGI platform right up until the switch over (or until you're happy your site is working on the new platform - if sooner). So you can take as long as you like getting it right behind the scenes before you set your own switch over date.