How to convert synced photo folders to camera roll folders?

There is no supported way to do that, for two reasons: the backup would grow huge, rapidly consuming iCloud space, and non-camera-roll photos are resized to display well on the iPhone while taking minimum space. As such, they're not the "original" photos any more. Backing them up would be a false sense of security.

The best solution is to back up the PC. The My Pictures folder will get backed up with the rest of your PC data, and you'll have copies of the original, non-resizd images should disaster strike.

There isn't anyway of moving all of the photos at once, but if you have one photo, you can click the edit button and a message saying "Duplicate and Edit" will show up at the bottom of the screen. If you click on that, the photo will automatically go to the camera roll. You can do like 100 a day and eventually you'll have all. Then, once your done you can un-sync them.

Sync your images via iTunes (all of them). When the album gets copied, go inside, Click select, click select all. After that, click the share icon in the left bottom. Click duplicate there and your stuff will get stored in camera roll. From there, go to iTunes on your MAC/PC and un-check the Sync Photos option. And sync your iPhone. You're done.

IOS 10.3.3