Not that bad, but will take a few hours. Drop the pan, remove the valve body, kickdown arm, NS switch and shift linkage. Care should be taken when removing the kickdown arm. Hold the arm steady while loosening the bolt or you will snap it on the inside.
After all that's removed, remove the nut that holds the manual shift arm in place, slide the manual shift arm out, replace the seal, and put her all back together.
Be careful when you put the valve body back in to insure that the internal kickdown arm is situated correctly and that the manual shift arm is in the ident of the manual shift valve.
Good time to add a shift kit while you're in there if it doesn't already have one. Also a good time to add a drain plug to the pan.