Change the schema without losing data

I need to change my schema: <describe — new column, renamed column,
split table, changed type>

Current schema: <paste>. Rows: roughly <n>. Downtime acceptable:
<yes / no>.

Write the migration:
- Forward steps, in order, each one safe to run on live data.
- Whether this needs to be done in phases so old and new code can both
  run during the deploy. If so, spell out the phases.
- The rollback, and be honest if there isn't one.
- What happens to existing rows — defaults, backfill, null handling.
- How long it will lock the table, roughly, at my size.

Then: what do I check afterwards to confirm nothing was lost?