From Beta 5 to Beta 6, I was able to overwrite only the changed files and run a few sql queries to do an upgrade on a test site. It took about 2 minutes to extract the changed files from the D6 package and create a zip file (about 950K in size) Another 2 minutes to upload the zip to the server and extract the files, overwriting the old ones, then 10 minutes tops, to run a few sql queries on the DB.

It may have just been a stroke of luck to be able to update the db that easily... there's no guarantee see more that it will be just as easy between B6 to B7.