I think the reason it worked with Original Alaric was because it was incorporated into the modified Vampirism spell from the start. It was one of the foundations of the spell. The way spells seem to typically work, in this universe, you can’t just added/remove a weakness or strength or modification to a preexisting spell (even if you created that spell). At best, you can take the spell and make modifications but said modifications can only work if the modified spell becomes another, stand-alone version of the old one. It won’t apply to or affect previous active version of itself. If it were that easy, Esther would have just changed the weaknesses of the Originals from White Oak to humans (like she did with Alaric and Elena). And that’d have solved all her problems.
That said, Qetsiyah, Dahlia, the Hollow proved that with enough power, you can remove the weaknesses/strengths/properties of an active spell.
