If I extract the legendary power from a legendary which orange affix roll within a certain range (e.g. The Dead Man's Legacy 50-60) but the roll is too low (let's say 51%). If I later loot a 60% version of the legendary, can I extract it again to upgrade the cubed version ?
There is no need to worry about this, the extracted affix is always at its maximum possible roll. With the Dead Man's Legacy, if you extract a 51% one, the passive you can slot will be 60% anyway.