On the positive side, there's a good chance for most of your sceneries to become totally useless for the new sim, given how the default world looks like

But sceneries like custom airports might still be useful, in some cases.
For the planes, despite what they said, I'm not so optimistic. Maybe some might work, but the thing is: most complex addons use complex gauges, systems and external modules (DLLs) that will probably not be compatible.
Simple planes like freewares might port more easilly, but I'm not sure if it would be worth the effort ? The contrast with the very high quality of the default planes might be cruel in some cases...