Oooh! I'd imagine the changeover would be possible but it could get very expensive -- if the factory would take it on at all.
We ordered with the twins and I'm thrilled with huge outside storage compartments. I've got the same small one you do next to the water heater. Then there is a 4.5' door over the battery compartment that gives access to about a 6' long compartment under the front window. And there is a small door under the roadside twin that gives access to something just under 4' of space.
Inside we've got standard doors with plastic bins under the twins. And we've got overhead storage across the front and over the roadside bed (similar to the storage over the dinette in back).
I'd do it the same way again. We've never liked the 25' side-to-side "queen" but our personal preference is for this length of trailer. There's nothing quite like the fore-and-aft queen in cosmotini's 28' FB International!