Transmutation Malleability
I'm not sure if this has been discussed already, just a thought I had.
Does it make sense that, instead of straight up transformation, transmutation can change the shape, but not the material, of real-life objects (i.e. malleability)? As a comparison, think of how transmutation works in Full Metal Alchemist. The user's understanding of the material(s) is crucial for the change, but they would need to carry the material around, or have it nearby somehow to use their ability. This may explain why we see more transmuters focus on altering the characteristics of their own aura instead, it's less cumbersome.
This could answer how Youpi can sculpt his body to produce spikes, his additional arms, or his wings. He's using the same material, but sculpting its shape. Then we have Bisky who has the same material (her body), but it's been compressed then sculpted to her cute form. With Youpi and his multiple eyes, this could be a weird gray area wherein he either uses transmutation to shape his body into something he already has (i.e. eyes), or he dips into conjuration to produce more eyes. Same for his rage form, it makes sense that he dips into conjuration to at least form that blade, maybe that head/shield form on his other arm.
How does this differ from conjuration? For two reasons. 1) Conjuration can create new materials; and 2) conjuration can impose rules to grant additional abilities to their objects. So, when we see all of those conjurers with transformation abilities, we can see how they not only changed the shape of the material, but create new material that didn't exist. For example, Biohazard, Fist Full of Weapons, Rider's High, as well as (stay with me here) the transformations of Crazy Slots. This dip can also explain things like Fun Fun Cloth dipping into transmutation to produce its shrinking effect (same material, different size).
I've never been a fan of the idea that transformation abilities are transmutation or conjuration. I felt that context mattered here as how what kind of transformation it is. I'd love to hear thoughts on this.