No – the curse adds to the damage of another spell, but does not replace the other effects of the spell
harm (Player's Manual) describes the damage he inflicts in two separate statements (it's me who points out):
In case of failure, it takes 14d6 necrotic damage, which is half the damage on a successful save. Damage can not reduce the hit points of the target below 1.
Confers the curse (Player Manual) can have the following effect:
While the target is cursed, your attacks and spells deal 1d8 additional necrotic damage to the target.
There is a bit of discontinuity between how the "damage inflicted" and the "damage taken" are used in these spells, but spread the curse very precisely increases the damage caused by "your attacks and your spells", which would increase the damage done, but would not affect the corollary "can not reduce the CV of the target to less than 1".
So RAW, I would say spread the curse would increase the damage roll for harm at 14d6 + 1d8, but otherwise the spell is unchanged. Note that this always makes the spell more efficient because the extra damage will also be taken into account to reduce the target's maximum health on a failed save.