Backwards compatibility usually means compromises on moving designs forward. I think the reworked P+ will have changes to the top cap, including the threading which means backwards compatibility isn't possible.
Current P+ owners have been lukewarm when it comes to preordering the strata. After 4 or 5 months they are still not sold out, which means there isn't a huge demand for current P+ owners. A few may miss the boat, but not enough to justify future design compromises.
My prediction is that the new P+ will be compatible with threading on the simplicity, making it laterally compatible, which I think would be a great idea.