I've found a solution that, while somewhat complicated, will allow you to move the Dropbox folder location on Windows 10 if you get the error "Some files cannot be moved" WITHOUT re-downloading your files. This requires admin level privileges.
First, open Windows Task Manager - you can do this by right clicking on the taskbar.
Now open the Dropbox Preferences and select the Account tab. The move location button probably won't work if you're reading this guide, but you might as well try it again at this point.
Switch to Task Manager and position the window so you can see both it and Dropbox Preferences at the same time. In Task Manager, select "explorer.exe", right click it, and select 'End Task'. At this point any file explorer windows will disappear along with the taskbar. You'll get them back later. Next select the Services tab and navigate to WMPNetworkSvc. This can be done quickly by clicking the 'Name' column to sort alphabetically, selecting any service in the list, typing 'w', and scrolling down until you see it. Right click on it, and select 'Stop'.
Now switch to Dropbox Preferences and try moving your location again. This should succeed. If so, in Task Manager, select WMPNetworkSvc, right click, and select 'Start'. Then press File -> Run New Task. Type 'explorer.exe' and hit enter. This should restart your taskbar.
If your problem is solved, you're done with this guide. Otherwise I have some troubleshooting tips.
[Troubleshooting] Download Process Explorer (http://filehippo.com/download_process_explorer/) and open it as an administrator. Press CTRL-F and search for "Dropbox". Many processes should show up. Each of these could potentially be the process that is keeping your folder locked. Start by stopping/ending explorer.exe and WMPNetworkSvc and try moving the folder in the Dropbox Preferences window again. Don't kill any Dropbox.exe processes. If you find a process that is keeping Dropbox locked and is marked hidden, you can kill it using the command prompt. Take note of the process ID (PID). Open a command prompt window as an administrator, and type "Taskkill /PID [your process id] /F". After killing each subsequent process, retry moving your Dropbox folder. After a successful move, reboot your machine.
