Get your co2 and dosing sorted out and make sure you have all the equipment before switching it out IMO. It's better to have it all and make minor adjustments then only having the lights and end up getting algae or melted plants. This is just by personal experience though and may not happen to you if you upgrade the light.
About the 24w, i'm not sure how long the bulbs are, I was saying 30" because my fixture itself is 30" since it fits over my 20g long tank. I would say the reflector fittings and the tubes would maybe be ~26" since there is a little bit of space on each side.