[chirp_users] Confusion about uploading .img file to radio

I have successfully downloaded, modified and uploaded memory settings. I thought that I could save such a file, and then use it for a subsequent upload if that became necessary. What I did:  I used what I had saved from a previous download, modified it, and then attempted to upload it.  I got an error: Failed to communicate with radio: Radio did not ack block 0

Can I only upload the same file that I downloaded?  If that's the case, why would one save a file?

Thanks,

Peter KO6R