Thailand/Siam has seen three capital cities already - Ayutthaya was destroyed by the Burmese in 1767, after reunited the country King Taksin established the new capital in Thonburi, and King Rama I finally moved it to Krung Thep (better known by the western name Bangkok), where as the first auspicious building he inaugurated the city pillar shrine (Lak Mueang, photo) on April 21 1782.
During World War II there were proposals to move the capital away from the busy overcrowded city. All happened during the first regime of Phibun Songkhram, at first in May 1942 the government decided to move the capital to Saraburi, the hometown of Phibun. I just read about this proposal for the first time in the 1942 book "Thailand - das neue Siam" (Thailand, the new Siam). As it is only mentioned with one sentence I have no idea what happened with that proposal, as it was obviously not become effective. Much better known is the second proposal of Phibun in 1944, when he planned to move the capital to Phetchabun and build a "buddhist city" at Saraburi. After he laid the foundation for a new city pillar in Phetchabun, the parliament failed to back his proposal and he resigned, but he returned to power after some years.