For instance, the city of Kingwood, TX was annexed by Houston. Can it ever be unannexed and become its own city again?
That all depends on how the people who control people with guns feel about it. One might believe that the Citizens of Kingwood would have some say in their being annexed by Houston, but one would be wrong. Even the large number of residents in Houston have limited effects on what happens to Kingwood.
If you think the residents of Kingwood have been shafted then think about the terrible annexation of Hawaii. And the subsequent filling the place with people who the natives dislike and US military to have them vote for statehood.