Why did burma change its name to myanmar and siam to thailand?

Question:Since 1989 the ruling military authorities in Burma have promoted the name Myanmar as a conventional name for their state; this decision was not approved by any sitting legislature in Burma, and the US Government did not adopt the name, which is a derivative of the Burmese short-form name Myanma Naingngandaw.

In Thailand: In Dec. 1938 Phibun Songkhram took over as military dictator, and the following year he changed the name of the country from Siam to Thailand.

Best Answer - Chosen by Asker: Since 1989 the ruling military authorities in Burma have promoted the name Myanmar as a conventional name for their state; this decision was not approved by any sitting legislature in Burma, and the US Government did not adopt the name, which is a derivative of the Burmese short-form name Myanma Naingngandaw.

In Thailand: In Dec. 1938 Phibun Songkhram took over as military dictator, and the following year he changed the name of the country from Siam to Thailand.

Burma changed to Myanmar because it has been under dictatorship for some years now. They were the ones who decided on the name change.
The other one I don't know, sorry.

Probably for the same reason Ceylon changed her name to
Sri Lanka. New rulers,new name.