Moving an entire list of tens or hundreds of thousands of contacts is a different job from a quick Backfill, so it has its own tool. Backfill runs from your browser and reads one batch. Migrate runs in the background.
Starting a migration:
Click Migrate on a bridge whose source is one of the built in autoresponders. WP Sync works through the whole list in the background, a batch at a time. You can close the page and it keeps going on its own.
Watching and controlling it:
A progress bar appears on the bridge row and shows how far along it is, with the counts of contacts done, sent, skipped and failed. Pause, Resume and Cancel are always available. When it has finished, the Cancel button becomes Clear.
Paced on purpose:
Autoresponders suspend accounts for sudden bulk imports, so a migration drips rather than floods. A very large list takes hours by design, and that is what keeps your destination account safe.
Nothing is lost and nothing is sent twice:
The position only ever moves past contacts that were genuinely delivered. If a batch is interrupted, for example by a timeout, a server restart or closing the tab, the next batch simply picks up from the same place. Anyone already moved is recognised and skipped, so you can stop and restart as often as you like.
Your Activity Log stays readable:
A migration records only problems, not a row per contact, so moving 100,000 people does not bury everything else. A summary for each batch goes to Diagnostics instead.
Choosing between the two:
Use Backfill for a list of a few hundred or a few thousand people. Use Migrate for a really big list, or whenever you want to leave it running and walk away.