Magical change of appearance in the lore

[Kakurady]

The existence of Fantasia is not acknowledged by dialogue, so it has no continuity with any other lore.

Outside of Fantasia, there are some cases of permanent changes to a being's physiology, but none of them appears to be fully controllable.

One tempered by a primal would take on features of that primal's element, becoming monstrous. Ishgardians who partake a dragon's blood may find themselves transformed into a dragon, usually one of three types. The Mad King Theodoric transformed his rivals into monsters. And the pixies also like to transform those who wander into Il Mheg—though other than leafmen, the results aren't fully predictable.

There are also targeted transformations used as temporary enfeeblements: adventurers may find themselves transformed into a toad, an imp, or an old crone, though usually they turn back before long—assuming they survive.

Finally when it comes to glamour, we as players use it to make one item appear to be another of a similar size and shape. But other users of glamour magick are not always beholden to such limitations.

The false Inquisitor was able to glamour himself into a dragon much larger than he is. And the sylphs are adept users of glamour, able to pass themselves as one of the five races, and even mimicking the Warrior of Light in the tribal quests. NPCs in the ninja quests also wield ninjutsu that allow them to pass as someone else; one antagonist scales himself up to face you.

Such uses of glamour do not seem to be common.

Nanamo doesn't use glamour to become her alter-ego, I wonder why?

[Iscah]

But in that case, your answer for 'how canon you want it to be' can't be 'not', because there's a bunch of things that are first divulged in Hildibrand but then become important later, like Godbert Manderville and the truth about Sil'dih.

Hildibrand is a story set in the canon world of FFXIV, so when it references other parts of the world, it is accurate to them.

That does not mean that Hildibrand is true and canon from the outside looking in.

Godbert and Sil'dih exist whether you have done the Hildibrand quests or not, or even if they were hypothetically wiped from the game entirely.

[Cleretic]

Hildibrand is a story set in the canon world of FFXIV, so when it references other parts of the world, it is accurate to them.

That does not mean that Hildibrand is true and canon from the outside looking in.

Godbert and Sil'dih exist whether you have done the Hildibrand quests or not, or even if they were hypothetically wiped from the game entirely.

This is true, but Hildibrand's also the place where a lot of stuff like that first gets expanded on. Big dumb slapstick it might be, but Hildibrand's tendency to 'look low' on the setting and do much smaller stories than we often see means that it shines a light on parts of the setting that just don't get talked about otherwise, because there's pretty much nothing to fight (or craft) about it.

That's basically what I'm trying to say: What Hildibrand says about the world is often something that Hildibrand's getting the first chance to expand on, so you can't just throw the whole thing in the bin of 'non-canon'.