[Dalillama]

I'd say that, since this spell is distinct from Lightning weapon, if you have a whip encahnted witih it, it's a normal whip until you activate the spell. When the spell is activated, you end up holding a whip handle from which a Lightning Whip extends, and the normal whip is gone for the duration, replaced by a string of crackling lightning. That said, if the whip had Accuracy or similar enchantments, I'd allow them to apply to the Lightning version as well as the original.