This is really a LEGAL issue and Realtors do not give legal advice:
Have you discussed the possibility of a new lease with your tenant? Wouldn't that be simpler?
It is up to you as the new owner to maintain the remaining lease period or to negotiate new terms with the tenant. You do not want to lock yourself into a lease that might be old, too low a price and does not take into consideration your investment costs. If for instance there is 6 months remaining on the lease you might consider keeping the price pr month as a courtesy to the tenant, especially if they have been tenants for several years and then extend the lease for an additional year or two at a new rent amount.
You will of course be obligated to pay a commission to that agent.