That’s drastic overkill. Such an early POD would have more than a few butterlies. For instance, a slightly more northerly route between the Philippines and the Americas could very well mean a discovery of Hawaii by Spain.
No, my idea was that by 1800, the US was still settling the Old West, and so by the time they pit their eyes on California, there will have been at least two generations of Hispanics born in California.
I agree on the US moving westward thing. so many people are Assigning mid 1800's mobility/power in late 1700's. by the 1780's, the US is barely crossing the apalachians. they're not trekking all the way to California and taking over the place.
a discovery at a time when the continent was barely discovered (by Europeans) and during the initial conquest/exploration makes the entire settlement/situation unrecognizable.
You can send an exploration up the coast/west California anytime you want, but if you want a realistic POD without a simple hand wavium, you have to go with the Bourbon exploration/settlement in the mid-late 1700's, and at that time, New Spain is barely making their way into California. What happens? Sure you're going to have a rush from Mexico, but as with the northern mining of Mexico/New Spain, this doesn't involve massive immigration. You'll have a lot of gold seekers, but no infrastructure, and no real communications with the populated parts of New Spain. Spain only has a tenuous hold on the region - it's mostly Spanish colors on maps. Other major powers, such as Britain, may swoop in and grab it.
The Russians were exploring/staking a claim to the region in that time frame, too, so they might be the ones making the discovery. Distance is against them, though, so it's going to be tough for them to hold on to it.