Sorta depends on which "box" you mean. Are you talking about the receiver "box" or the transmitter "box"? If you mean the receiver, it may be ok to switch the crystal. I say "may" because some manufacturers have different crystal models and you need to be sure the crystal you have is the proper model for the receiver. If you're talking about the transmitter "box", the answer is no, it's not ok to just swap crystals. The FCC regs require that transmitter crystals only be swapped by people with the equipment and knowledge to check the output and make any necessary adjustments.