Amazon's photo storage service boasts unlimited photo storage for Prime members. Over time this can of course result in more photos in the cloud than a user can easily transfer to another service.
The only option today for moving photos from Amazon to a different service is to begin by downloading, and hope that uploading to another service goes well. Some users may have challenges with storage or bandwidth. Photos can easily be 2MB each, requiring a GB of storage for each 500 photos. It's worth using specific bulk export tools rather than using the regular Web page interface for photos, because bulk export tools have been desgined to require less back-and-forth and complete more reliably.
Amazon support recommends using Amazon Drive as the interface through which to download photos. A 3rd party article suggests using the Photos interface directly. In either case, it does not seem possible to do a single bulk download/export of photos, and instead the user must select multiple files and photos and hope to select and download them all eventually using some methodical process.
This article would be improved with some more information about what the export looks like and whether metadata is completely lost. Contributions welcome!
The source code (markdown) for this article can be found on GitHub.