I am trying to connect to a version 8 database in a clean v10 Umbraco, to allow the wizard to do the upgrade for me, but I run into this error

Don't hotswap databases, it will confuse Umbraco and its indices and caches I think.

I used a clean installation that was already set up with a clean new database --- How do I remove those?

It's an appSetting. Default is disabled

Are both of those in the Data TEMP folder?

I think so. Delete the whole

umbraco\Data\TEMP

Right, done - Then what?

Make sure all kestrels and iisexpresses are shutdown, connectionstring the 8abase, F5

Inspect the entire callstack, and all Umbraco logs

I think this goes beyond my competence, as far as I am aware there's only a callstack that refers to a lot of "external code" but it starts and ends in the main function - And there are no errors in the logs from Umbraco

Maybe I am misreading this but, I believe the thing we recommend in the docs is: - do a clean v10 install - stop the v10 site - change the now existing connection string (in appsettings.Development.json) on the v10 site to the v8 database - start the v10 site again - you should now see the upgrade wizard - provided that unattended upgrades are not configured in the v10 appsettings It might actually be beneficial to set unattended upgrades to true, I've seen one problem being avoided by that recently (can't remember exactly what). Note: make sure that the v8 database is on the very latest: 8.18.14

I created a new project based on the 10.latest template, and created a database for it to go through the installation process with, I then closed the project to a production-based database from a v8 project that is currently live (So that database works), and when I start the project with the V10, connected to the V8 database, I run into the above error

Another option is to use uSync - Install uSync on v8 site, export everything. Then create a brand new v10 site with db, install uSync and import the previous v8 export. Maybe less paid unless you need all the v8 history

Yeah I can't quite tell from the error what might be the problem. Indeed, as Simon says, my next option would be to use uSync to export from 8 and import to 10! 👍 It's a very excellent tool.

Does the database have a string for version?

Kinda. It's a string for which migration has been performed last, in the UmbracoKeyValue table. It's a migration GUID though, you'd need to look at the Umbraco source code to figure out which GUID corresponds to which version.