Is it possible to upscale a windowed game?

Fullscreenizer solution

One possible solution is to download Fullscreenizer (direct download), which is a free, portable, and open source tool for Windows. Using it, you can "Fullscreenize" a game (or any application), which forces it into a borderless window, stretched to fill your entire display.

It also has some other features which make it a better solution than native borderless windowed modes, such as preventing taskbar flickering and preventing popup notifications (optional).

How to use Fullscreenizer

You should set the render resolution to windowed 1080p (or your desired downscaled resolution) in your game and then "Fullscreenize" it once you've locked in your settings.

If you don't see your game or app on the list of programs, click "Refesh" in Fullscreenizer to refresh the list of running programs.

Why Fullscreenizer?

Games are inconsistent with how they handle fullscreen and borderless on lower resolutions, so that's why for this case we should rely on an OS-based solution and not a per-game solution.

Another upScaler is the IntegerScaler.

Download, start application, focus desired window to be upscaled, press Alt F11, the window app/game transform into a FullScreen application.

Only drawback, it can only upscale in integer steps. So window to be upscaled must be half or quarter of full screen size, windows with different proportion will lead to black blocks on sides.