My current understanding is that in the short term only a few older kits will disappear totally, most kits will be simply repackaged, and a small amount will go DS only.
Longer term there are a lot of new kits to come out, so I'd imagine some older kits will then be retired (empire knights I seem to recall being named specifically), and as I am told the new aesthetic is quite different I doubt there'd be much call for some of the older kits anyway. Latest I heard (and I don't hear a lot these days) was that the main push for the opening months will be...... drum roll..... yep Chaos & Humans, so you can probably expect that several of the older kits from these ranges will be the first ones to go, and stay gone. What I find interesting is the amount of terrain I'm hearing is looming.... seems unprecidented especially for WFB, although I also hear that the kits are very modular so many of the tings could be combinations of kits rather than standalone kits in their own right. When the new edition comes into full swing I expect many more kits/ranges will then become much harder to buy, as some ranges I am told will not even go DS only, they'll just go.
