Unfortunately, your version is too old to qualify for the upgrade pricing. You will have to purchase the full license.
Additionally, since you are moving from 6 to 10, you will need to convert all of your .fp5 systems to .fp7 format when you upgrade. This process can be fraught with issues (I migrated 278 systems from fp5 to fp7 during '09). You may want to investigate New Millenium's tool called Metadata Magic. It can be indispensable when faced with a lot of upgrades to do.