To access Atmos soundtracks, the audio signal must be Dolby Digital Plus 7.1 or 5.1 to begin with. Anything that's just Dolby Digital will never have Atmos encoding.
That said, Atmos is a joke for the most part. Studios are rarely paying to put a human at the helm to master Atmos (or DTS:X) soundtracks. Instead they just run the 5.1 or 7.1 soundtrack through Dolby Surround (or Neural:X if the working master is DTS) to get fake immersive sound, then they encode that as Atmos (or DTS:X) so you think you're getting something good, when you aren't. There are a SMALL number of 4K UHD discs released with GOOD Atmos (or DTS:X) soundtracks. You'd waste a lot of money on finding those titles without a resource too help you focus on the discs that have GOOD Atmos or DTS:X soundtracks... a resource that scores or otherwise provides some value judgement on each immersive soundtrack. The studios know that even if they make something now, they can sell a remastered version to you in 5 years that might finally deliver on the promise of immersive sound.
One immersive sound movie I had to write a review of had EXACTLY 5 seconds of sound in the height channels over the entire 1 hour 50 minute movie! I'm not sure why this reality isn't more widely known, but I've been writing about it from the very first Atmos and DTS:X discs for thousands of subscribers. Nobody else seems to take the immersive soundtracks seriously. There is a scene in the last Star Trek movie where hundreds of small alien ships are piercing the hull of Enterprise, then prying the hull open so the occupant can get into the ship. There's gunfire (well phasers or whatever), hand to hand combat, yelling, explosions... and during all of that sound, the ONLY THING you hear in the height channels is "Red Alert" several times. NOTHING ELSE. That is just stupid... a massive fight like that would echo up and down the halls and there would be constant ambient sound above your head if you were there during the fight... but nope, all you get is "Red Alert" a few times. Use Auro-3D/AuroMatic on the same soundtrack, and there's near-constant ambient sound in the height channels that blends perfectly with the lower level speakers.
And... because there are so many BAD Atmos and DTS:X soundtracks out there, you can actually get BETTER sound from those movies by using Auro-3D on them instead of Atmos or DTS:X. Of course if your pre/pro doesn't have Auro-3D, you miss out on the best upconverter for immersive sound that exists... by a long long shot. Auro-3D now has a single setting for immersive sound. Earlier, there would be an Auro-3D option for discs with Auro-3D soundtracks (popular in Europe), and an AuroMatic option for upmixing the original 2, 4, 5.1, 6.1, or 7.1 soundtracks up to 9, 11, or 12 channels. If it was a bad Atmos soundtrack, just use the Auro-3D setting on it and Atmos or DTS:X will be bypassed for the 5.1 or 7.1 lossless track that is then upmixed with AuroMatic without you having to manually select AuroMatic as you did in the past. So if you are considering a processor without Auro-3D, think hard about that, I find AuroMatic to be the FIRST AND ONLY upmixer that makes stereo music sound better than a really good stereo system can make it. AND it makes bad Atmos or DTS:X soundtracks (I'd guess 95% of discs with Atmos or DTS:X as of Nov. 2019 have horribly bad soundtracks that would be improved massively by using Auro-3D and forego-ing Atmos or DTS:X. If you rate the immersive quality of the 3 upmixers (Dolby Surround, DTS Neural:X, Auro-3D/AuroMatic), I would start by giving AuroMatic a "10", not because nothing will ever be better, but because today, if I had only one immersive sound option, it would be Auro-3D/AuroMatic. Play the same content with DTS Neural:X being sure to match levels, and I would be very surprised if anybody would rate it higher than a 4 or 5. And Dolby Surround would likely get ratings of 1 or 2 for consistently making everything you use it on sound worse from stereo to 7.1.