It was not unusual in the past for changes in sovereignty to still respect the prior established private property rights. For instance both the Louisiana purchase and annexation of California promised in law to respect existing Spanish land grants. So while this was the case in legal theory, these rights were not immune to corruption.

I grew up on land in the Bay Area that formerly belonged to the Ranchero de las Pulgas, and now I live in the east bay. I don't remember the name of the Ranchero here, but much of it had already been sold off to yankee traders before 1846. In both cases after annexation the widows controlling the remaining land were ripped-off by shyster lawyers. IIRC in one or both cases the widow's own lawyer! This despite the fact the family of Ranchero de las Pulgas had thrown in with the American cause during the Mexican American War.