AC LEDs were introduced by companies such as Seoul Semiconductor and Lynk Labs. They consisted of string of LEDs in parallel with an LED string of reverse polarity, packaged into a single device. High-voltage AC LEDs — with a Vf of at least 170V (which requires about 40-50 LED chips in each string) — can be placed directly across 120V 60 Hz line voltage. The end result is that AC LEDs use the LEDs themselves to rectify the AC line voltage. This approach is used in Seoul Semi’s first-generation AC LED Acriche line, but not its second-generation Acrich2. AC LEDs have problems with photoscopic flicker as well as an inability to dim off of a phase-cut triac dimer switch. (AC LEDs should not to be confused with newer AC drive LEDs.)
A ballast is what the lighting industry calls the power supply that is in every fluorescent fixture. Fluorescent lamps require a different voltage from ac house voltage — just like your other household electronics, such as a desktop computer or video screen. The job of a ballast, or power supply, is to take the ac house power and convert and regulate it to a voltage and current that will cause the fluorescent lamp to turn on and continue emitting light. LED lamps also need a power supply, but they are built into the lamp itself, where they are referred to as the driver electronics.
Older ballasts on T12 bulbs were often magnetic ballasts, which are much less power-efficient than the more modern electronic ballasts.
Although the mechanical portion of the fixture is the same for a T8 lamp and a T12, the ballasts are different. If you simply want to upgrade your existing T12 fluorescent fixture so that it can operate with T8 lamps, you’ll still need to change the old ballast to a new T8 electronic ballast. But why not re-wire the fixture to remove the old ballast completely, and run retrofit LED lamps?