Assuming you are using AES with CBC, best case you will have to resend every block after and including the first one that changed.

My understanding is that CBC is among the most secure forms of AES encryption because it is essentially impossible to have patterns in data (unlike EBC). Practically speaking then one must assume that it is common to upload most of a file. Any software that boasts this security cannot effectively 'diff' your files.

EDIT: Formatting