We've had as-definite-as-computerly-possible assurances that the exisiting items will work with P5, just probably not with the new features (or at least not fully). Since it will take a while for creators and users to learn the new stuff (compare out-of-the-box P4 with DAZ stuff, or good free stuff like Sergemarck's or Kozaburo's) there will be, at least, a period following launch when you are likely to prefer working with P4-style legacy items to the new features.