The free plan does not expire. The free plan will reset the leftover sending limits on the date you started your account each month. So if you signed up on Aug 20, you would be reset each month on the 20th. You will not lose any information; it will remain stored in your account.
If you go over 2000 subscribers, you will be asked to purchase a plan, but you will not lose any information in the account or access to the account between when you go over the plan and when you make a purchase. Sending will be temporarily suspended until a payment is made, but once a payment is made full sending access will be restored.