From the Release notes: Tome Upgrades can now be found in random treasure. These wondrous items will allow you to upgrade an existing inherent bonus in an ability score to the next higher inherent bonus. Upgrades that turn a +4 Inherent Bonus into a +5 Inherent Bonus will be able to be found in treasure levels of 27 and higher.
Explain to my how this upgrade is any different than receiving the tome itself. If I have a +2 tome and recieve a +3 tome upgrade, how is that any different than receiving a +3 tome?