Pre-purchase Midnight. How does it work?

Hi, I have never purchased an expansion in wow. Actually, I just payed for a month subscription like a year ago. Just to play classic.
But I really want to start playing the game. But i dont know how the expansion system works.
Right now I can pre-purchase the Midnight expansion. But if I buy it now, can I play the expansion that is up rn? or it would work as if I just bought the subscription?