Move it to a different stack

I have <current stack> and I want to get to <target>. Here's the code:
<paste or describe>

Do NOT rewrite it all at once. Give me a migration plan where the app
works after every step:
- What moves first, and why that piece.
- How the old and new parts talk to each other during the transition.
- The point of no return, and what I should verify before it.
- What I could keep on the old stack forever without shame.

Then we do step one, and only step one.