No, different wattage of flourescent bulbs have different lengths. You cannot change bulbs like incandescent bulbs.
Depending on if you want low light plants (generally plants with darker greens), or light demanding plants, you should target for a total wattage of 50W or above for low light tank, OR, 100W or above for light demanding plants.
Even though total wattage is just a guideline, but even for low light plants, your lighting is still not enough.
If you decided to go with light demanding plants, besides lighting upgrade, you will also be needing CO2 injection and fertilization to help plants grow.
If you really are not a DIY kind of guy like me, then I would suggest you measure the length of the tank and go out buy a new light fixture that will fit your tank. But if you want to save $$$, you will have to do the upgrade yourself.