I heard that Rabbi Dunner of London advises to keep as long as possible the Aravot of the Lulav during Sukot since a mitzva was done with them, one should maintain it and use it as much as possible. Is that correct?

Remah (O.H, 654: 1) maintains that one should change the Arovos every during Chol Hamoed. This was the minhag of the Divrei Chain and Nimukei Orach Chaim. However, many Poskim would not change the Arovos daily even when they could . (Minhagei Chassam Soffer, Belzer Rebbe, Orchos Rabbenu 2: 296, mentions that the Chazon Ish would change them from two to three days. See Piskei Teshuvos 654: 2).