Ya you will need a different ballast and different end caps for the lights. I have never retro fit T12 for T5 but someone here I'm sure has. I use a Sunlight Supply 8 tube one.
Puts out 40,000 lumes and makes 20" bushes in a little under 4 weeks from clones w/low heat. I then pop w/HPS and get great results. I also have 100 watts of T5's in with my 100 watts of LED and the T5's make the led veg room expload.