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.

Hey Boss,

It looks like you might have copied the cms directory over the top of the old one, without removing old files?

SS 3.5.2 doesn't contain a cms/reports/SideReport.php file as you can see here: https://github.com/silverstripe/silverstripe-cms/tree/3.5.2/code/reports

I suspect your issue is that SideReport.php is hanging over from the 3.1.5 codebase and needs to be deleted - but I suspect you'll get a lot more of these errors unless you remove all the old code and entirely replace the framework, cms etc. folders with the new versions.

Hope that helps! ☃️