hi @joebrew
Collect treats forms as "updated" if the form definition or the media files are different on the server than they are on the device. It won't redownload media files it's already got on the device however as it compares the hashes of local files with the hash the server returns for the media files and will skip files it already has. This optimization requires the server you're using to implement the Open Rosa spec correctly of course - Central does this, but a custom server might not. If you're using "Exactly Match Server" for form management, this should all be happening in the background.
extracted from post When are media files re-downloaded?