Most of the time, all you need is a new ballast.
Check the label on the ballast, to see if you need "shunted" sockets. If so, then be sure to wire-nut both sides of each socket to the new leads to emulate shunted sockets. The original sockets that are used for T-12 bulbs can be used with the T-8 bulbs if they are in good physical condition, since the spacing of the pins on the tubes are the same.
Usually we.replace a 4 banger with a 2 banger including a reflector.
Pardon my ignorance, but what is a "shunted" socket?
RC
This likely is the way to go. It's also a good time to move to T5's. You might want to take some light meter readings on the existing fixtures and run some calcs on what you really need. You then can make a decision to move to T5's, regular or HO.
For anything other than residential, I have moved to T5HO, either with a step ballast or a dimming ballast (preferred). You can get fixture efficiencies over 90% now!
RC
In all fairness, T8 is more energy efficient, less expensive, and more reliable...