Can I upgrade existing T12 fluorescent fixtures to T8 just by replacing the ballast and bulbs?
Replace ballast and lamps, and for extra savings, on a four tube per bay fixture, get a 3-lamp ballast and run only three tubes.

The extra efficiency of the smaller tube plus high-frequency electronic ballast means that three T-8 tubes can put out nearly the same light as four T-12 tubes.

The ballast tends to be a lot smaller, and weigh much less than the t-12 ballasts.



Wiring varies -- your ballast will have a wiring diagram.  But to give you an idea, here is an example of wiring for an instant-start electronic ballast (which are easier to wire, but tend to fail bulbs faster); and a rapid-start or programmed-start electronic ballast (which often matches the old magnetic ballasts): 



For the smaller 18AWG solid-core wires in ballasts, don't use the common yellow/red wire nuts.  Use blue wire-nuts, or for instance, push-in wiring connectors such as Gardner Bender PushGard, others are available... make sure the size is correct for 18 AWG wire. 




You can reduce number of bulbs, since T8s are brighter.  A complete building rewire from four T12 to three T8 tubes per fixture eliminated 38 tubes and at 32W each, was a savings of 1216 Watts in power reduction if you only count tube elimination and not also ballast efficiency over mangetic (est. 10-15w per ballast replaced). PLUS, NO FLICKER! For someone with fast vision, not seeing 60Hz flicker in the peripheral vision was almost worth more than the power savings.