Detect change to user_meta and retrieve old and new values

I figured out the answer. I needed to use the insert_user_meta filter, not an action hook. And then I ALSO needed a combo of get_user_meta (to get the old data) and $_POST to get the value being changed to. Putting it all together, here is my final code to find when a particular user_meta field (in my case created with ACF) changes from one value to another:

add_filter( 'insert_user_meta', function( $meta, $user, $update ) {
  if( true !== $update ) return $meta; // if not an update (b/c it is a create) do nothing
if(is_admin()) { // check if we are in admin not front end
  $old_meta = get_user_meta( $user->ID );
  if( $old_meta['verified_member'][0] !== $_POST['acf']['field_5ad4eecd7564b'] ) {
    error_log("verified_member was modified from " . $old_meta['verified_member'][0] . " to " . $_POST['acf']['field_5ad4eecd7564b']);
  }}
  return $meta;
}, 10, 3 );

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