In general, the Add option does the followings:
Update (overwrite) the contents of existing media files from BitPim to the phone if they're different (or more accurately: if the file sizes are different).
Copy new media files from BitPim to the phone (those files that exist in BitPim but not the phone).
In general, the Replace All option does the followings:
Update (overwrite) the contents of existing media files from BitPim to the phone if they're different (or more accurately: if the file sizes are different).
Copy new media files from BitPim to the phone (those files that exist in BitPim but not on the phone).
Delete all media files from the phone that are not in BitPim.
BitPim Online Help built 17 January 2010