Rewrite negative number into positive numbers
For Drupal 7 you could use the Global: Math expression field to perform a mathematical operation. Load the initial field first, suppress it from display and use the replacement token within the Math field: (abs([initial_field])).

For Drupal 8 this field does not exist (yet), though there is a sandbox linked in post #10 here: https://www.drupal.org/node/2544670. It might work.