One of the things I notice about a lot of maps of Mexico and Central America when annexed by the US is that the old state lines are maintained. I find this somewhat unlikely. Partially depending upon the POD being before these state lines have solidified or been established and partially because the US would want senators and representatives differently distributed. Post any maps you feel are relevant.
For example, the Yucatan would likely become a single state so that it can be more like the republic of the Yucatan. Baja would likely go to California or South California. The border lines with Arizona and New Mexico would likely not be as straight and would for that matter be radically different. There likely wouldn't be a New Mexico, just a Mexico. The large number of states in the middle of the country would likely be combined into a few larger, more populous states.
Another question is about Central America assuming that were to get annexed as well. What would happen to small states like El Salvador or Belize?
This is sort of a spin off question: how would the US defend the border with Columbia compared with how it defends the border with Mexico OTL?