Not ideal, to have multiple lists.
Instead, here’s what we did:
1) set the autoresponder to have a delay of, say, 2 hours.
2) Enable it, which dropped all the existing peeps into the funnel.
3) In phpMyAdmin, ran an UPDATE query to set all rows in the table wp_newsletter_autoresponder_steps to step=1 and status=1. (Status of 1 is Completed as far as the autoresponder is concerned; Step 1 was technically unnecessary, but it felt complete, since these people in fact all did get the email originally.)
4) After verifying that no current user was in the autoresponder funnel any more, set the autoresponder back to no delay/immediate delivery.
Seems to have worked.