How did West Virginia split from Virginia?
Besides that, California is NOT a state. It’s a Republic. So is Texas, for that matter.
If Texas was readmitted to the Union along with the other Southern states following the organization of postwar civil government, the treaty that brought Texas into the Union would be abrogated. However, if Texas or the other Southern states did not have the right to secede, which was Lincoln’s contention, then the treaty remains in force and Texas could be split up without Congressional approval. This would be an interesting issue for the Supreme Court.