Yes, it down grade from insider to preview. If you current backup you will be able to get back most of your settings. I don't remember having to reload any of my data. Just had to run updates to get to os level I wanted.
That's not the right thing to say,
In the release notes you find this:
REMINDER: We have changed the backup format for Windows 10 Mobile devices to reduce the size of the backup stored in OneDrive. As a result, if you do a backup on a device running the latest Windows 10 Mobile Insider Preview builds and move back to the released version of Windows 10 Mobile (Build 10586) and restore from your backup – your Start screen layout won’t restore and remain the default Start layout. Your previous backup also gets overwritten. If you need to go back to Build 10586 temporarily, once you are on Build 10586 you should disable backup so it doesn’t overwrite the good backup from Windows 10 Mobile Insider Preview builds.
I was not on the level that changed backup formats before I updated the firmware with WDRT.