Can I use bulbs I already have in my new stringer?

You can use existing bulbs you already have; you just would need to confirm the base sizes are the same and that you either have all LED bulbs on the strand or all incandescent. If you combine incandescent with LED, you lessen the life of the LED bulb because you are feeding it with more energy than it needs. An incandescent bulb needs 5 watts of power to work, the LED needs less than a watt to shine.