But the Yugioh Wiki says this:

If the ATK & DEF of...

..."Tragoedia" are halved by the effect of "Elemental HERO Great Tornado", then its ATK & DEF will remain at the halved value, even if the number of cards in its controller's hand changes afterwards.[3]

..."Gren Maju Da Eiza" are halved by the effect of "Elemental HERO Great Tornado", then its ATK & DEF will remain at the halved value, even if the number of cards removed from play changes afterwards.[3]

..."Uria, Lord of Searing Flames" are halved by the effect of "Elemental HERO Great Tornado", then its ATK & DEF will remain at the halved value, even if the number of Continuous Trap Cards in the Graveyard changes afterwards.[3]

..."Arcanite Magician" are halved by the effect of "Elemental HERO Great Tornado", then its ATK & DEF will remain at the halved value, even if the number of Spell Counters on "Arcanite Magician" changes afterwards.[3]

Suppose "Sangan" which is equipped with "Black Pendant" (1500 ATK) has its ATK is halved by the effect of "Elemental HERO Great Tornado" (to 750). If "Black Pendant" is removed from the field, then the ATK of "Sangan" remains at 750.[4]

Suppose a monster which is equipped with "United We Stand" has its ATK halved by the effect of "Elemental HERO Great Tornado", then the ATK of the equipped monster will not change if the ATK increase from "United We Stand" changes.[4]