Thailand or Siam as it then was known was never formally colonized, unlike its neighbours Burma, Laos, Cambodia, Vietnam and Malaysia. If it had been, what would the impact have been on Bangkok?
To answer this question, it is necessary to look at the changes made to other capital cities, such as Saigon (Ho Chi Minh City), Phnom Penh, Vientiane and Rangoon (Yangon). These cities were all subject to colonizing forces, the first three under French influence and the last under the British. These all became colonial capital cities (Saigon was a regional capital city rather than a national capital city, because Vietnam under the French was for long divided into three parts). What are the characteristics of a colonial capital city? First, it should be located on the coast because it was sea transport that represented the principal means by which goods accumulated in the colony could be relocated for consumption in the imperial centre. So, the capital of Burma under the British was moved from Mandalay in the north to Rangoon on the southern coast.
Second, the city centres were divided into zones depending on residence requirements. Superior land would be reserved for the colonizers and those privileged clerks and servants of the ruling class. The native people would be restricted to particular zones away from the great monuments and symbols of power. Further, according to the French pattern of urban development, the areas in which the poor lived must be properly regulated and easily accessible to the police and other state authorities: so, as in Phnom Penh and Vientiane, a grid system is created and comparatively wide streets or boulevards created to join individual blocks. This made it much more difficult for political dissidents or criminals to hide from the state and its representatives. Riots and demonstrations can be contained or suppressed as required.
Finally, new classes of intermediaries would be created: the clerks, police, civil servants and others through which the imperial power can control a much larger population of local people resentful of foreign control. The British used Indian troops and Sikh and Nepalese Gurkha police; the French used Chinese or semi-Francophone Vietnamese. These people were given special privileges and acted as a buffer between the local people and the Europeans: if there was violent resistance, it was mostly aimed against these intermediaries.
So, in the case of Bangkok, there would have been no need to move the location, since it is located next to a port. However, the main commercial and government centres would have been moved nearer the port; possibly the royal institutions would have been relocated to a peripheral area where they would have appeared less important. The higgledy-piggledy sois and sub-sois that now characterise Bangkok would all have been eradicated and a grid system superimposed upon the ‘native’ Thai areas. Between these two parts of the city would have been an intermediate zone dominated by the Indian, Vietnamese or Chinese (probably Indian) people imported to administer the colony and which would now be running the country rather than the Thai aristocracy. These people would have been the recipients of violent protest, almost certainly, in the post-colonial period. Depending on how the independence process was handled, which is an imponderable question, a successful Communist revolution would have been more or less likely.