Closing on my first 6-unit apartment building soon! What happens to existing lease?

[Jake Thompson]

I'm a noob, but I'm trying to learn all I can! 

All units are occupied and the previous owner's leases are yearly and all of them renewed not long ago. I'm needing to raise rent a decent amount ($200 is the highest, for one of the tenants) to get closer to comparable rates. I have a lease I would like to institute, but do I have to wait for the existing leases to expire to raise the rent, or can I add an addendum to the existing lease to do that, and then replace the actual lease with my new one down the road?

Thank ya in advance!