Rank 10 Chocobo Reached!

[Maelwys]

Pics/vid or it didn't happen. I'm not risking gimping my choco to test this.

Look in the OP.

At timestamp ~5:50 he has hit Rank 10 and picks a HP boost from the Attacker Tree, his Chocobo (in Healer Stance at the time) immediately gains the benefit of it.

The OP initially has a +10% HP trait from the Defender Tree, and a +5% HP trait from the Healer Tree.
This puts him at 2235 HP (115% base if all traits stack)
So his Chocobo's base HP is potentially 2235/1.15 = 1943.478 HP

He then picks a +5% HP trait from the Attacker Tree.
This puts him at 2332 HP (120% base if all traits stack)
2332/1.2 = 1943.333 HP

Proof Positive.

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You can even try it yourself:

Have you got any HP traits in your current chocobo, in any stance tree? (the first one is Rank #3 in each of the three trees)

Yes? Good.
Check the number on its HP bar.

Now put the chocobo into a stance that doesn't have the HP trait on its corresponding tree.
Check the number on its HP bar again.

If HP traits weren't active across stances, those would be different numbers.