For me, I think, it depends mostly upon why and how the Fantasia is used. If someone has a genuine IC reason to use one beyond the "Oh noes, by drink was spiked!" I think it can lead to some very interesting RPs. Of course, this is also dependent on the change not just being the player wanting to change the character's design.
It may be poor form here, but I'm going to use myself as an example. I'm making the switch with Rin IC to an Au Ra, but not for my own reasons, but for strictly IC ones. She's blind, and grew up for a long time in Doma, so she's familiar with the Auri. It recently dawned on her, while trying to find a way to assist her slowly deteriorating aethersight that the Auri horns may help her at least in short range talks and such, as they assist in spacial awareness.
So, she's since been trying to find (and recently found) someone who may be able to brew up something for her that may assist in the change. Whether it's technically a Fantasia is up for debate I guess, but it'll make for some very interesting RP.
TL;DR - I think it entirely depends on the reason and RP surrounding the Fantasia use.
It may be poor form here, but I'm going to use myself as an example. I'm making the switch with Rin IC to an Au Ra, but not for my own reasons, but for strictly IC ones. She's blind, and grew up for a long time in Doma, so she's familiar with the Auri. It recently dawned on her, while trying to find a way to assist her slowly deteriorating aethersight that the Auri horns may help her at least in short range talks and such, as they assist in spacial awareness.
So, she's since been trying to find (and recently found) someone who may be able to brew up something for her that may assist in the change. Whether it's technically a Fantasia is up for debate I guess, but it'll make for some very interesting RP.
TL;DR - I think it entirely depends on the reason and RP surrounding the Fantasia use.
(Forever a work in progress)