Try a climate control system calibration to clear the lights. There are several ways to do that listed in this link. Hope one works:
http://www.allpar.com/fix/codes-climate.html Only perform these procedures if your A/C is working, otherwise, lights will be flashing till you fix the A/C.
Yup. Sounds like you know dropping the trans pan to change the fluid only changes about a third at best. Another change wouldn't hurt. Consider buying a pump and suck it out the dipstick.