Well, the quickest graph is this (after JK has been patched to support post-256x256 textures):
* 256x256 can be instantly replaced with a 1024x1024/2048x2048/4096x4096 texture without any tiling/internal duplication.
* 256x128 can't be upscaled with a 512x256/1024x512/2048x1024 texture, because it will mess up the texture coordinates in JK. So in this case, for example, you have to turn the 1024x512 upscaled texture into a 1024x1024 one where the 1024x512 one is repeated twice. So it'd require the use of editing software (Photoshop etc.), and could likely be automated.
As for how the maths go for smaller textures, well, that's where my brain power stops.
But in the 64x128 one above, the hi-res version that you made (256x512) had to be repeated 8 times in the 1024x1024 texture to make it appear properly in JK.