The term is not changed automatically. If you offer a free trial for 7 days, then you should set a price for renewal. Then, if you allow clients to change term, they can switch to monthly. If they don't switch they'll be billed for 7 days at a time. Or, you can keep an eye on all these trials and change the term to 1 month yourself, then they'll renew for 1 month instead of 7 days.