I just finished retrofitting nine 8' T12 fixtures to hold six 4' T8 LED lamps each. It's not difficult.
I had the same problem. Rather than use one or the other I used a mix of both types. Now, in place of 18 T12 fluorescent lamps I have 18 4' 4000K T8 LEDs and 36 4' 5000K T8 LEDs.