Is it possible to change the local sync folder for Amazon Photos clients on Windows 10 PC
I realize this question is a couple of months old, but I figured I'd share what I know in case anyone else has this question.
You can change the local sync folder in the Windows 10 client—but it's not entirely intuitive. Basically, you first need to disable synchronization (Preferences: Sync: Disable sync…) and then reenable it. When you do, you'll get the initial setup screen which will give you the option of changing the sync folder. You may need to restart the Amazon Drive application between these steps.
As of Version 6.0, you can now change this to any drive. Previously, you were restricted to your C:\ drive. The folder name, however, remains restricted to "Amazon Photos".
If you have a large amount of data, it may be worth your time to first copy your files to the target location; that way, Amazon Drive won't need to download them again. It will still need to verify that they are the same files, which will take some times, but at least you won't be tanking your bandwidth. (You could instead move your files—but, if you do, you'll want to do this after disabling sync, but before reenabling it; otherwise, Amazon Drive will interpret these as deletes.)
Obviously, this process leaves some room for improvement. Ideally, Amazon Drive would allow you to change the sync folder without first disabling synchronization, and then move the files for you so that it doesn't need to validate them against the server. Hopefully, a future version of the client application will add in this functionality.