First, may I welcome you to Tropical Fish Keeping forum. Nice to have you.
Now to your question. The "watts per gallon" so-called "rule" is something to forget. Plants need far less light than many will tell you. I have less than 1 watt per gallon on all but one tank, and I have no trouble growing plants as the photos under "Aquariums" below my name on the left illustrate.
However, you must decide on a few things in order to have success. First, what sort of planted tank are you expecting? And with this, what sort of paraphernalia do you want to spend all your money on?
My tanks are quite low-tech, natural systems. Minimal light (sufficient to allow the plants to photosynthesize) out of deference for the fish, and added liquid fertilizer to balance. On my 115g which is 5 feet, I have two 48-inch T8 tubes, amounting to 80 watts (more or less, some tubes use less watts to provide equal intensity). If your present tubes extend full length, or nearly, in a double parallel row, you're probably fine. You may have to change the tubes, I don't know what spectrum and kelvin you have; we can discuss this later.
T8 and T5 are not compatible; you would be better buying a new fixture. However, I would not. I had a T5 dual-tube with two 48-inch HO tubes over my 115g last year, and after one week I took it back for a T8 dual-tube. It was far too much light. If you had NO tubes, it would work, but as they are basically equal to T8 the added expense seems pointless.
Hope this gives a bit of help. You might want to have a read of my article "A Basic Approach to the Natural Planted Aquarium" in 4 parts at the head of this section of the forum. A general background to how all this works, with the photos of my tanks showing the expected results.