After it becomes a creature, Nantuko Monastery will still have all its abilities.

Once you activate the last ability, it doesn't matter what happens to the number of cards in your graveyard. Nantuko Monastery will remain a creature that turn even if that number falls below seven.

A noncreature permanent that becomes a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature. Notably, if you turn Nantuko Monastery into a creature on the turn it entered the battlefield, you won't be able to tap it for mana.