Yes it will reset the boost.
Its description from PS!:
Speed is doubled on held item loss; boost is lost if it switches, gets new item/Ability.
Unburden doubles the Pokémon's Speed stat when the Pokémon's held item is lost. If there is no held item to begin with or if a held item is traded for another, Unburden will not be activated.
This is the
Hope I helped.