Channel 51 means channel 51. Some Hitec receivers will work with JR radios. It will have to be either an auto shift or shift selectable receiver as the standard Hitec receiver is negative shift and JR is positive shift. (if it is shift selectable make sure it is set for positive shift for your JR TX)
Shift is a term for how the radio handles the signal (not to get too technical)
Dual conversion? Your receiver gets a signal from your radio and converts it to a usable signal for your receiver and eliminates erronious information. Dual conversion means it proccesses the signal twice giving it a cleaner signal to transmit to the servos (again not too technical but you get the idea)
If you get a dual conversion receiver make sure you also have a dual conversion crystal for that receiver.