Buying a 17-Unit in Phoenix, Do I Need New Leases at Closing?

[John Phillips]

I'm looking at purchasing a 17 unit apartment building in Phoenix, AZ. The property currently has existing leases and tenants in place. At what point should I have new leases with the tenant? 

Should I let the existing leases last until the end of each tenant's term and then draft new leases or should I draft new leases as soon as I close/purchase the property? 

Thanks BP