If you still have T12 lighting you should definitely convert to T8 or possibly T5.   There isn't just one option when you consider retrofitting from T12 to T8.  T8 lights are brigher than T12, which in some applications is great.  In other applications the extra light from T8s is not needed or wanted.  In these cases you should consider the using the low watt ballasts.   The mainline manufactuers all make a LW ballast. 

These low watt ballast have a lower ballast factor, typically around .77 compared to a .88 normal light ballast.   The low watt ballasts use less wattage and will drive the lights less than a normal ballast, resulting in lower light levels.  With the LW ballasts you can use the 800 series lights and still get approximately the same light levels as T12s, but you get in excess of 40% savings compared to approximately 33% when using normal light ballast.   One of our favorites is to use the low watt ballasts with "super" (high lumen) T8 5000K lights.  This gives a much more natural bright look and reduces energy levels on a 4 lamp fixture approx 43%

Other options for T12 retrofits are reducing the number of lamps, while using high output ballasts and "super" T8 light bulbs.  By using 2 T8s in place of 3 T12s you can get same light levels, and reduce maintenance costs significnatly because you are only changes 2/3s of the original number of bulbs.

 The consumer switching to T8 lighting from T12 really has a lot of options.  They can choose to maximize light levels, energy savings or trade off somewhere between the two.  We provide a free evaluation of your lighting, cost to retrofit using different options, and an energy savings estimate.