Purchasing a duplex with existing tenant lease

[John Guion]

I recently purchased my first duplex that i will be occupying and renting out. The property currently has tenants in the top level of the up and down duplex with a lease expiring later this year. I plan to occupy the lower level leaving the sizable basement open. The lease the tenants hold currently has a section for use of the basement until the property is sold at which point the new owner(me) may add a fee or terminate use.

The situation gives me a few options and i'm not sure how to proceed. 1. If they continue to use the downstairs as an in home office id like to charge them a fee for the use but i'm not sure how i would add this into writing. 2. Take the basement over completely. 3. Just allow them to continue uses the space until their lease expires in a few months and put this new space into their formal lease. 

The upper unit rents for well below what it should so i was hoping to bring in a little more money until the rent can be increased with a new lease.  Any suggestions or insight would be helpful.