The fire element will remain when transformed, but you can add (with proper gems) other damages from different elements like Arcane and Bolt. For instance: you can get some best cursed gems that improves: 1° effect, phy. dmg +23,9%; 2° positive effect, add Bolt dmg +13,5; and 3°effect the negative one. In this case, in normal mode you deal phy. dmg + bolt dmg (due to the gem). In transformed mode, you deal phy. dmg, fire dmg and bolt dmg. Anyway, I suggest using phy. and fire dmg only.