I recently purchased a 91 Accord. The S light came on and it locked in park. I changed the transmission fluid, which was in bad shape. It seemed to fixed the problem. A few days later, the transmission started acting up. I changed the fluid again. It now shifts hard from 1st to 2nd. I have been told that some cars require that the transmission be re-trained by going through a sequence of shifting, after having serviced the transmission. Is that the case in this car, or is the transmission going out?

 

if the 'S' light came on there should be a code stored, jumper the service check connector and check for a flash code.
http://members.troublecodes.ne...n.jpg
As far as it being 'locked' in park that would be a shifter/shift interlock or linkage problem.
FWIW a lot of first time Honda owners are surprised at the way they shift. The idea of positive pressure shift is that the clutch discs will wear less.