Apparently it is possible. I learned this because my new TV supports Atmos signals. Even though my receiver only supports 7.1 I was able to select Atmos as spatial sound in Windows. I was surprised that the Atmos signal worked on the 7.1 receiver, but the answer to why is pretty simple.
Atmos is delivered not as a codec, but as an extension to True HD that gets folded into the bitstream to maintain backwards compatibility with older gear. When a compatible decoder is detected, the Atmos extension will be processed. When a non-compatible decoder is detected, the extension data gets ignored and you hear just the regular 5.1 or 7.1 Dolby True HD soundtrack.
And it’s a good thing they did it this way, otherwise Atmos blurays might have needed to include two versions of the soundtrack, waste of space. This method ensure compatibility.
Dolby Digital Plus, a codec used by video streaming services such as Netflix and Vudu, also supports Atmos. The same process at work with True HD applies here: the Atmos extension gets folded into the DD+ bitstream, and is either decoded or ignored depending on your receiver’s capabilities. A key difference between the DD+ and True HD codecs, however, is that DD+ uses lossy compression.
So all that extra data is just gone. Fucken Dolby