Change to resolution after picture taken

Sorry, can't be done. Those pixels weren't recorded, so there's no way to regenerate them.

Think of it this way. Suppose you look out the window and see a red car drive by. Look out the window an hour later and maybe a blue car drives by. Now, how many white cars drove by between those two cars? If you weren't looking (or had a surveillance camera), there's no way to tell. In your picture you've got a red pixel over here and a blue pixel over there. There's no way to tell what was in between.

There are some tricks that can be done to interpolate. If you have a bright red pixel and next to it a dark red pixel, then the pixel between them probably was a medium red. But maybe not: there could have been a blue object in between. Or the pixel in the middle might have been the same color as one of its neighbors. There's just no way to be certain.