Let Siam formed by Ayutthaya be eligible for the White Elephant achievement

[Jespoke]

Ayutthaya's mission tree turns them into Siam.
Siam is not eligible for the White Elephant achievement.
The achievement already requires you to start as Ayutthaya, so there is no harm in letting Siam complete it too.

 

[RMS Oceanic]

See also: Lotharingia from Burgundy getting Burgundian Conquest.