replacing just the bulb is not going to do much. Florescent bulbs of a fixed length and diameter have standard wattage. The only thing adjustable is the sprectrum (or more accurately, color temperature) but your bulb is fine for growing plants.
I would recommend getting a dual t8 fixture if you are looking for long term success with low and medium light plants. 15W over a 20 is barely adequate for most low light plants.
I would recommend getting a dual t8 fixture if you are looking for long term success with low and medium light plants. 15W over a 20 is barely adequate for most low light plants.