Actually, the snippet I sent above is not doing what I want. I just found this snippet on github showing a more stable way of replacing this entry by removing it and adding a new one.
;; Refile will set mail to All Mail (basically archiving them). I want this to
;; auto-mark them as read, so I redefine refile to add the +S tag.
(setq mu4e-marks (assq-delete-all 'refile mu4e-marks))
(push '(refile :char ("r" . "▶")