I guess what's confusing on this one is it's not like a normal upgrade. It's actually an import just like importing to a new product. The database structure is completely different so it's actually taking all of your old data and importing it into the new tables.
if you have access to a new database that's probably the easiest and safest way as you can just nuke the database if you run into issues and want to start fresh without needing to worry about touching your old tables from 6.5.5.
