The easiest way I can think of is to use layers. The absolute simplest, though not necessarily the most 'real', is to find a photo you have of a nice, big sky, then copy your Epcot photo, and paste it as a new layer over the sky photo. You can then pick the blend mode of choice such as 'darken' which will replace the lighter areas of the Epcot photo with the darker blue sky behind (though any other area of the photo that is brighter will also be replaced, like white t-shirts, so you may need to use the eraser tool to do some erasing of the areas you don't want to show through), or simply leave it on 'normal' blend mode, and erase the sky from the Epcot photo with the eraser and lasso tools. It sounds complex, but can usually be done in 10 minutes or less, unless you've got lots of seriously fine detail against the sky like a big leafy tree.
You can also use a gradient blue background to do the same thing, just letting some of the color 'tint' the white in the sky...but you can't use a solid color or it will look fake.