Why is most of the stuff in the marketplace not compatible with unreal engine 4.9?

I have a question. If your pack is just stuff like assets (meshes or materials), and shouldn’t have been affected by the new release, but it hasn’t been flagged as compatible on the Marketplace with the new release, can the user download and use it? Will it open in the new version of the engine? Do you need to have the version of the engine the pack is at to convert to the new version?