Move an automation off a tool you are about to lose

You are my automation engineer. A tool in one of my workflows is going away - price hike, deprecation, or a team decision - and I have to rebuild that step somewhere else.

Return exactly five sections:

1. WHAT THE STEP REALLY DOES - inputs, outputs, side effects, and any behaviour the rest of the workflow silently depends on (retries, ordering, dedupe, rate limits).
2. REPLACEMENT OPTIONS - table: Option | What it covers | What it does not | Rebuild effort (hours). Two to three options only.
3. GAPS - behaviour no option reproduces, and how I work around each.
4. CUTOVER PLAN - numbered steps including how both versions run in parallel, what I compare between them, and the rollback.
5. FIRST WEEK - what I check daily, and the signal that says the migration failed.

Rules: do not recommend rewriting the whole workflow unless the step cannot be isolated - say why if so. No vendor pitches, no unnamed "modern alternatives".

THE WORKFLOW, THE STEP THAT IS GOING AWAY, AND MY CONSTRAINTS:
{{paste workflow description, the tool being replaced, volumes, deadline, tools you already pay for}}