After you reveal a non-Pivot Maneuver on your Maneuver Dial, you may immediately add +1 to your speed.
When an upgrade says that you may do something than that "something" is optional. So since Warhorse uses "may" it's effect is optional.
Rulebook page 29:
A card ability is only considered optional if it uses the word “may” OR has an “Action:” or “Attack:” header OR requires its owner to disable or discard the corresponding card (or add [ ]s to it) in order to activate the ability.
Regarding the dificulty of the maneuver I'd say it stays the same as the originally chosen maneuver.