That's not strictly true in all cases (and definitely not in the 3 you put together)
in the case of Burma to Myanmar, this happened in 1989, long after the military coup in 1962.
Ceylon changed to Sri Lanka at the time it became a republic (from 1955 - 60 it was a protectorate and then I think a dominion like Canada -- need to check that)
The reason for the change is the term Ceylon is derived from Sinhala -- the majority folks there. By changing to Sri Lanka, this was to be more inclusive of Tamils I'd guess. However, the Tamils were already at a disadvantage from 1956 when the Sri Lankans made only one language -- Sinhalese. Sinhalese is an Indo-european language and Tamil is a dravidian language -- the scripts are different and both are definitely anything BUT mutually intelligible. So the problem started before the name change
The Ivory Coast is and has been majority French speaking so their official name always was Côte d'Ivoire. I think they asked for the name in English to be Côte d'Ivoire not the English translation in the 80s. Their problems started over a decade later. In short -- they never had an official name change.
If we call Burma “Myanmar” and Ivory Coast “Côte d’Ivoire”, why don’t we call Germany “Deutschland”?