[OnePlus 3 Guides, News, & Discussion] GCbard:
I spent some time on live-chat with oneplus support to be sure how to downgrade to marshmallow. In short they mentioned that you need to have a special remote session with oneplus support to make that happen. Here is a copy of my chat with them. I wanted to share that with you all...

2:49:50 PM - undefined said : Please wait one moment...

2:49:53 PM - Erik said : hi

2:49:56 PM - Rosevil said : Hello Erik. My name is Rosevil how can I help you?

2:50:04 PM - Rosevil said : Thank you for taking time contacting OnePlus Customer Support. I am happy to help.

2:50:11 PM - Rosevil said : Hello Erik.

2:50:21 PM - Erik said : hi

2:50:37 PM - Erik said : i just downloded the latest community buid

2:50:40 PM - Erik said : beta 8

2:50:45 PM - Erik said : excited to try it out

2:51:09 PM - Erik said : in the post where oneplus posted the build it was stated the following....

2:51:43 PM - Erik said : After upgrading to Android N, you will not be able to directly downgrade back to M builds. Our customer service team has a special build of Android M that will allow you to downgrade, but this will format your device. If you'd like to flash back to M, please contact our CS support here: [https://oneplus.net/support] Please keep in mind that this is beta software. This build will not be as stable as our official OTA releases. Given that this is a shift to a brand new version of Android, there may be more bugs in this build than previous Open Beta builds.

2:52:02 PM - Erik said : can you provide me with that special build please?

2:53:19 PM - Erik said : are you there?

2:53:55 PM - Rosevil said : Thank you for the information Erik. If you want to downgrade your OS from Nougat to Marshmallow, I will endorse you to our Level 2 specialist and book you a schedule for a remote session, is it okay with you?

2:54:15 PM - Erik said : a remote session? what does that mean?

2:55:12 PM - Rosevil said : Remote session is our level 2 specialist will help you access your phone and will assist you to downgrade your OS to marshmallow.

2:55:41 PM - Erik said : i was hoping to have the build in hand and then perform that task myself

2:56:11 PM - Rosevil said : I understand, what we can offer you is the downloads available in our website.

2:56:19 PM - Rosevil said : http://downloads.oneplus.net/

2:56:21 PM - Erik said : i see those

2:56:25 PM - Rosevil said : Please try to check this.

2:56:27 PM - Rosevil said : I see.

2:56:43 PM - Erik said : but I cannot just simply download the latest stable to downgrade

2:56:45 PM - Erik said : can I?

2:57:14 PM - Rosevil said : I am afradi that we are not able to provide any instruction or steps for you to do it by yourself since our Level 2 specialist as well is not allowed to do so.

2:57:34 PM - Rosevil said : As adviced, offer you our download files in the website.

2:57:43 PM - Erik said : lol

2:57:51 PM - Erik said : here is what I am asking

2:59:05 PM - Erik said : i understand how to load community builds and then downgrade back to stable builds via adb sideload. I have done it many times. The issue with this current new beta 8 nougat community build is that it was posted that I need a special build of stable OOS in order to downgrade back to stable from beta 8.

2:59:27 PM - Erik said : from the post from oneplus i was told that i can get that special build from customer support

3:00:34 PM - Rosevil said : I am currently contactin one of our Level 2 specialist here, pleasew bear with me.

3:00:41 PM - Erik said : ok

3:06:01 PM - Rosevil said : Hi Erik. I already talked to one of our Level 2 specialist, as I gathered information from him. I understand that you know how to flash your OS knows a lot on hot to do it by yourself, but we or only our level 2 specialist here has the tool to downgrade to you marshmallow version via remote session with them.

3:06:27 PM - Rosevil said : how*

3:06:45 PM - Erik said : really?

3:06:51 PM - Rosevil said : Yes.

3:06:53 PM - Erik said : why is only a remote session possible for downgrading

3:07:18 PM - Rosevil said : Because our level 2 specialist has thier tools here to do it.

3:07:29 PM - Rosevil said : This is the information I gathered from them.

3:07:39 PM - Erik said : can he/she provide us with the tools so that we can do it ourselves?

3:07:55 PM - Erik said : does it involve using sdk?

3:08:06 PM - Rosevil said : I am sorry but they are not allowed to proivde it to you as this is an internal files from us.

3:08:37 PM - Rosevil said : As much as they want to provide it but this is an intermal property/files.

3:08:42 PM - Rosevil said : Hope you understand.

3:10:07 PM - Erik said : Pelase update any postings of the community build on the oneplus community making tha clear. Most people will use beta 8 thinking they just need to get a special build from you. if they know they need a special "remote session" to downgrade that will deter people. They should know that information.

3:10:36 PM - Rosevil said : I will send your suggestion Erik.

3:11:00 PM - Rosevil said : I do apologize and thank you for understanding.

3:11:37 PM - Erik said : thank you

3:11:48 PM - Rosevil said : You're very much welcome. It's my pleasure to assist you.

3:11:54 PM - Rosevil said : Is there anything else that I can assist you with?

3:12:01 PM - Erik said : no, thanks

3:12:12 PM - Rosevil said : Have a wonderful day. Take care and be well.

3:12:16 PM - Rosevil said : Thank your for contacting OnePlus customer support , have a wonderful day. â€œAn automatic survey will pop-up on your window when you click the "Close" button on the lower left corner of the chat screen. It is just a 1 question survey that will let you rate me and the service I have provided to you today. We appreciate your feedback. Thank you and Never Settle.