Chain of events:
Spain does better in the War of the Pyrenees and doesn't sign the Second and Third Treaties of San Ildefonso.
Louisiana remains in Spanish hands.
Shortly after, due to growing financial difficulties and the realization that the US could take New Orleans but may be willing to buy it, Spain sells New Orleans and all possessions east of the Mississippii to the US.
Plauasble?
What happens?
I expect that eventually the US will take a large part of the rest later by force.