This is not the unemployment office. This website is totally independent of any government office. I do my best to give accurate answers but I do not have access to facts that may alter my answer and have to rely solely on what you provide. From what you have said, you are being told that one claim based on older earnings will expire in 18 weeks. Apparently you have had earnings since filing that claim so when the current claim expires, they can start a new claim based on more recent wages.
If the federal extension is available in TN when your current claim expires, you may be able to go to that and receive benefits at your current rate. Be sure to ask at that time.
As far as what happens to the balance on an expired claim - the state may decide to use it to pay you in the future but they are not allowed to extend the life of a claim simply because there is a balance in that claim.