To make an Atmos mix the multi-tracks of the recording are needed, I don't think any upconverted stereo tracks will go through pretending they are Dolby Atmos mixes.
I hope this format will succeed but it depends on all the headphone listeners out there, if they prefer the Atmos version over the standard stereo track, the format will have a real chance to make it. This can be like a "trojan horse" for us super-HiFi-nerds to finally get a surround music format that will survive this time, unlike SACD which depended on a 5.1 setup that most of the listeners will never install in their homes.
This time around it's an object-based format that will work in both headphones as well as a multi-channel system, the format will use all the physical speakers you have in your Atmos setup no matter how many they are.
Some good things come with the format as well, like the loudness limit of -18 LUFS that will prevent crushed and overly limited mixes. And according to people who mix in Atmos, there is also less need for compression on individual instruments in the mix now when they have a lot more space and don't need to cramp everything into a stereo mix of just two channels.
If you have 4.5 hours at hand, I suggest you see this video with Andrew Scheps and his friends discussing mixing in Atmos and what they think about the format. There's a lot of good info in there.