Although you can't start a battle with an already Terastallized Ogerpon,
Anubis was kind enough to test editing the Ogerpon's form during the
middle of the battle. Here's the clip:
https://streamable.com/843iv7
In this clip, Anubis starts with a regular Teal Mask Ogerpon (Ogerpon-0). She then edits it to be Ogerpon-Teal-Tera (Ogerpon-4). Notice that Embody Aspect does not activate when sent out like this. She then edits it to be Ogerpon-Hearthflame-Tera (Ogerpon-6) and Terastallizes it. It becomes Ogerpon-Teal-Tera (Ogerpon-4).
So that means a few things:
- Embody Aspect should not activate unless the user is Terastallized.
- Hacked Ogerpon-[form]-Tera forms should be allowed to Terastallize.
- Hacked Ogerpon-[form]-Tera forms will become whatever the Tera Type is for that Ogerpon.
Expanding on these tests with my own testing, Embody Aspect doesn't work unless the user is specifically a Terastallized Ogerpon. The game still softlocks if you try to Terastallize Ogerpon with a non-Grass/Fire/Rock/Water Tera Type. However, it will allow, for example, a Tera Water Ogerpon-Hearthflame holding Hearthflame Mask (Ogerpon-2) to Terastallize to Ogerpon-Wellspring-Tera (Ogerpon-5). Doing so will change Ogerpon to Water-type, but still give an Attack boost, and Ivy Cudgel will still be Fire-type.
So the developer fixing this should implement their fix with these mechanics in mind. We obviously aren't going to allow softlocks though, so I would prevent any Ogerpon with a non-Fire, Grass, Water, or Rock Tera Type from Terastallizing in the battle as a compromise.
EDIT: Here's some
cart footage of Tera Water Ogerpon-Hearthflame holding Hearthflame Mask (Ogerpon-2) Terastallizing. I suspect now that it is not actually becoming Ogerpon-5 (Wellspring-Tera), but rather Ogerpon-6 (Hearthflame-Tera), and simply has the visual effects of Ogerpon-5.