Let me be honest here I have not tested this myself. My MiA1 was on Oreo stock and then I moved to /e/ and have been using the /e/ ROM for 7+ months without any issues. Some users who were on pie had issues when they downgraded for e.g read The pie upgrade changes the baseband version on the phone. There are links here of details how some folks got it to work.
Right, thanks for your feedback. I’ll read later after work. Is /e/ planning an upgrade to pie any time soon?
Soon, no. But Pie development is in preparation.
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It sounds a bit dodgy as you have to restore someone else’s efs backup. Is that okay to do with a different phone?
Actually it seems that I have an old TWRP backup from August 2018. I might give that a go.
I had used the official stock oreo ROM for MiA1 9.6.4.0 or some version from that point . It should be available on XDA or MiUI sites.
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Hello Yander2m.
I faced to the same issue. When I received my Mi A1 I updated it to the last version of android. When I tried to install /e/ I had everytime a boot loop after the /e/ logo.
I tried several time the solution proposed by /e/ (seel above) but it did not work. After several days I rolled back my Mi A1 to oreo with this solution : https://myphoneupdate.com/how-to-downgrade-mi-a1-android-pie-to-oreo/ and it works. Now everything is fine, I’m a happy user of /e/ 
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Thanks for sharing this @Thomas
@Freddie in case you try the method as mentioned by Thomas in his post please let us know if it was successful. We will have the updates added to wiki as a help to users who want to downgrade from Oreo to pie.
I gave up in the end. I tried reinstalling an Oreo twrp backup of my own but my phone wouldn’t boot. I’ve switched to another device for /e/. Thanks for the help
But now I received an email saying pie is on mi a1. Downloading!! @Manoj which one is the pie download?
Hi @Freddie /e/ ROM’s are not available on Pie as yet. We have just started the upgrade of the code base. Should take a month (min) I guess for it to be build-able.
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Hi, I have the opportunity to buy a second hand Mi A1 with Pie or a refurbished Mi A1 (I guess it comes with Nougat?). What do you think would be the best thing to do, buy the one with Pie and wait for the Pie /e/ version to be released in order to avoid problems, or buy the one with Nougat, update it to Oreo and the install /e/? Is it even possible to update it from Nougat to justo to Oreo, without having to jump automatically to Pie?
The pi/e/ build for tissot or MiA1 may take a couple of months I guess. We just started the code base upgrade for Pie. The MiA1 stock ROM is nougat.The way I went was my stock nougat build I upgraded to stock Oreo version 9.6.4 and then came to /e/. The reason why the stock oreo version was old is because there were certain firmware upgrades made to the later versions of Oreo which made the switch to /e/ difficult for some users.
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Thanks @Manoj So if I understand correctly, the safest thing would be to update from Nougat to max. Oreo . 9.6.4 and from there install /e/. I guess I’ll have to flash this 9.6.4 version, as I supposed that the phone (in the settings menu) won’t let me choose which Oreo version I want to update, right? By the way, anyone was succesfull in downgradin from Pie to Oreo? (because of the supposedly anti-rollback system…)
You are right . The system will directly pick up the latest updates and you will eventually end up with Pie. Which will be a problem to roll back. Not sure if LOS 16.0 also works is you come from Stock Pie on MiA1.
Ok, thanks. Do you know where I can find the 9.6.4 oreo version to flash it? And if I get a phone with Oreo, (for example Oreo 8.1) there’s no problem to flash an older Oreo version (ex. 8.0) right?
Thanks @Manoj. Unfortunately, the refurbished one as well as all second hand mi a1 have already updated to Pie, so I 'll have to leave the flashing for later, when /e/ develops Pie, so I can flash it from a Mi A1 with pie without major issues.
Guess you are right to wait @graz. The chances of messing up the revert to previous versions are too great. Hold on for a couple of months the Pi/e/ ROM for tissot should be out.
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