You don't have to reprogram the fob after changing battery.
Maybe ur fob is on it's last leg. Autozone usually have Radio Freq detector laying around for customer to test key fob and see if they are any good. If it test out bad, you need new fobik. If tested good, you might have to visit dealership for programing. I believe you need at least 1 good key fob to program extra new key fob without Scan tool.
If you are looking for alternative, you can always get Aftermarket Alarm system with Remote keyless entry function.