do value <- whenJust mSomething pure
complicatedActionWith value
As @tdammers noted here, it will depend on what the previous programmers (and now yourself) consider to be important: in some places using the case-expression could be more informative; in other places: not so much. But I would tend to leave alone those places where detailed “walk-though” documentation exists - the writer of that documentation (and possibly the associated code) would have chosen that more verbose style for a reason…