I can't offer assistance with LED other than to say, make sure you get a good unit if you do. But I have used T8 (basic fluorescent) for 20 years and still prefer it (though a good LED unit would tempt me, if I could be certain). I don't know your level of knowledge on this, so pardon me if I repeat some of what you already know.
First, T8 tubes come in basic standard wattages for each length, so a 24-inch T8 will be 17w (used to be 20w but with the improved manufactured tubes they are more energy efficient with less power). Watts is simply the measurement unit of the energy needed to light the tube; it is no indication of intensity as this depends upon the phosphors in the tube. So there is no point in rebuilding the fixture if you stay with T8. However, there are stronger tubes. I have a single T8 over my 29g and 33g tanks, and I use the Life-Glo tube which is the most intense light in the tube size that I have been able to find. I will take a guess that you might have an Aqueon tube that came with the unit? These are next to useless. However, a single 24-inch tube over a 40g is really pushing the envelope, but depending upon your plant species this could be sufficient. I don't know how long you've had the tank and plants, but if the swords are surviving now it might be sufficient to use a better tube.
If the tank dimensions limit the tube length to 24 inches, you better option might be to replace the workings with T5. This light is HO (high output) and equivalent roughly to 1.5 T8 tubes of the same length and spectrum. I think one T5 would be better than two (one often sees tubes in twin) which would be way too much light.
Spectrum is also important. Plants need blue and red to drive photosynthesis, but studies have shown that adding green to the mix does improve the plant's response (all else being equal), so the "daylight" tubes with a Kelvin between 5000K and 7000K are in this range. The Life-Glo I mentioned is 6700K. You can get tubes like this in T8 and T5 (they are not interchangeable in the same fixture).
Byron.