This is tough. If the grudge is intangible then I'm not sure how the Angels can grasp it to go back in time with it. If thay can some how take it back in time will it only be for the duration that the grudge has been a ghost? Or is it possible that thay can take the grudge back in time to when it was human and the erase its humanity as well?
Hmm... interesting... the best I can gather, the curse possesses or haunts people, which means it would need to do such to the weeping angels. However they cannot be killed, so when the curse attempts to it would essentially be forced to try and possess the angels. Angels not only kill people, but they can actually turn people and objects into people by either going through the eyes (doors to the soul) or because anything with the image of an angle itself becomes and angel. That means the curse, while trying to possess the Angels, would itself be turned into an Angel.
So I think ultimately this would result in the curse becoming an Angel.