It doesn’t look like there’s a lot of blue sky there to key (but I didn’t bother to see how much was in the Raw file to pull out – since the upload was compressed). So you might be better to do a roto of your “skyline” since its a locked off shot. Then put the sky in you want and crush the levels of everything else down. The tree is an issue, so you will have to crush the levels to pull a matte and combine that with your roto. Keep in mind as K mentioned above, those objects tend to be darker than the sky so forget the matte and you could crush the levels just a little and then multiply/blend the tree etc right over the sky with no matte at all.
Notice that parking sign. Unless you have a car head light on a sign at night, its usually in silhouette against the glow of the sky – and all skys glow unless you are waaaaaay outside of town. So you might not need a pulled matte at all.
Cinema tends to use a blue cast to trick you into thinking its night (the opening scenes of Jaws are day for night and blue blue blue). So you can use sodium vapour pools color blended into the shot over a general blue cast and this gives a classic warm/blue coloring we expect in night shoots.
If you really want to get tricky, they tend to water the streets down for night shots to get interesting reflections. Again this is something we have come to expect in film so the more of those kinds of things you can add the more you fool the viewer into accepting the old tropes.
The white objects are an issue as they are going to get muddy as you try and darken them enough. Make a roto and use the roto as a multiplied element for a silhouette of those objects. There is very little detail in night time, er, details.
Shadows at night are actually more contrasty than the ones in the daytime (the blue daytime sky adds some fill to lighten them up) So don’t be afraid of contrast as you darken down.
You can also add some noise over all. CCD’s of todays camera get pretty noisy in the dark and high ISO films used for night shooting are the same (Amadeus used only practical lighting on film – the opening scenes are very grainy) and we’ve come to expect it (just like the totally unrealistic blue cast from the moon). The noise will also help tie the fake sky into the real footage.
Adding some offscreen flares (calling Mr. Abrams, Mr. JJ Abrams…) can create a fog that will also tie different elements together. It also helps even out the black levels which can really scream “fake” in Day for Night.