I touched on this in an above comment[1] (after you posted so I didn't expect you to see it), but essentially because of how AES CBC[2] encryption works, an encrypted block relies on the previous block to correctly be decrypted. Uploading and replacing blocks that are different will not work; the next time that file is decrypted every block after the first newly uploaded one will not be correct.

[1] -- https://news.ycombinator.com/item?id=7787791

[2] -- http://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#...