The next time you upload a file - like a locally produced book, or serial issue - you may get an error message from your FTP client. I've attached an image of one client's error message.
We migrated our file storage books.klas.com to a new site last evening (Wednesday, April 9). It's possible that you'll get a message indicating that the stored Host Key you have for books.klas.com doesn't match the one you get when you connect now. That key changed with the new server, so this is an expected message. You should feel free to update the key with the new value.