Let's be honest here and acknowledge that Disney would never spend the money to just change MK's Splash and this whole project is happening in Florida only because, in their minds, they're splitting the cost between two versions.
MK's Splash changing is simply a matter of convenience and brand consistency. They do not care one bit about the differences between the two versions and why they were not built exactly the same to begin with. You could change the theme of MK's Splash to something that better fits its surroundings, but that would cost extra and this is a park that has abandoned attraction, retail and dining space because the most popular theme park in the country wants to avoid any kind of investment.
The New Orleans theme is being pushed because it fits at Disneyland, which is the only thing that matters to California-centric WDI. MK is the junk park in the swamps and they know that park's fans are so starved for anything new they'll be happy with whatever they get.