Temporary UI should still have a point of view. The current sidebar rail is not the final object, but it establishes the width, the gesture, and the idea that navigation can live at the edge instead of across the top.
That makes the later sidebar migration less mysterious. The future design can replace the panel content and visual treatment while keeping the viewport-level behavior already in place.
Prototype the behavior first
The exact menu content will change. The mobile behavior may change too. What matters in this pass is learning whether the rail feels natural and whether the page can handle navigation without crowding the identity mark.
Once the behavior feels right, the visual version from the other project can arrive with fewer unknowns.