How does the HP boost for Terapagos work in battle?

Question

Terapagos's stellar form gets a buff to its base HP stat. How does this hp stat work when an already damaged Terapagos terastallizes into its stellar form? Does its hp increase based on missing hp or scales with the increased maximum hp? (How I phrased this may be unclear but I'm just asking how the increased HP stat works in battle). Please correct me if I've got how Terapagos transforms wrong, I've never actually used the turtle before.

Comments

I think Zygarde is the only other Pokemon that can change HP mid-battle.
"the difference in HP is added to Zygarde's current HP[...]"
https://bulbapedia.bulbagarden.net/wiki/Zygarde_(Pok%C3%A9mon)#Trivia

Answer 1

Same as Zygarde, or leveling up, or evolving; the new HP is always full. It retains the damage taken not the HP remaining.
Example: A max HP Terapagos has taken 50 damage (344/394 HP) after Terastallizing it will have 474/524 HP.
Basically, whenever a Pokémon's max HP increases [except when it Dynamaxes] its current HP increase by the same amount.

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