The new mexico territory was part of Spain then Mexico before it became a territory in the mid 1800s.

I'm doing some family genealogy research and I was curious as to how this process occurred for what has been reported as tens of thousands of citizens who became part of a new nation over night.

Was citizenship granted when they were admitted as a territory or was it later when they became a state in 1912?

Are there records or rolls of the individuals who went through the process?