For consistency with other mlp character tags (and their aliases ):
princess_cadance -> princess_cadance_(mlp)
princess_cadence -> princess_cadance_(mlp)
And while we're at it:
mi_amore_cadenza -> princess_cadance_(mlp)
princess_mi_amore_cadenza -> princess_cadance_(mlp)

Why was it ever spelled "Cadance"? The credits said "cadence" and it matches the musical theme too.

SirAntagonist said:
Why was it ever spelled "Cadance"? The credits said "cadence" and it matches the musical theme too.

According to mlp.wikia.com, it's spelt Cadance and only credited as Cadence in The Crystal Empire. Not sure how reliable of a source mlp.wikia.com is, though.

http://mlp.wikia.com/wiki/Forum:Cadence_or_Cadance%3F

Looks like Cadance is as certain as anything can be without a big Hasbro sticker on it.