I have just tried upgrading from 3.5.2 from 3..1.5.
The actual website seems to have been upgraded fine, i can access the website, go around it, and it all works really well... Except... when i go to /admin i get database errors, and when i do /dev/build i get errors about class conflicts, things like:
Fatal error: Uncaught exception 'Exception' with message 'There are two files containing the "SideReport_EmptyPages" class: "/var/www/sites/redacted/cms/code/reports/EmptyPagesReport.php" and "/var/www/sites/redacted/cms/code/reports/SideReport.php"' in /var/www/sites/redacted/framework/core/manifest/ClassManifest.php on line 568
I also get these errors when i try to ?flush=all.
The way i tried to upgrade was to replace the delete the folders cms, framework, vendor, reports and siteconfig. then copy in the new folders.