Crowns don't often change color so it would be good to know the cause of the discoloration as that can be indicative of a definite need to redo crown. Is it more in the root or gums at the edge of the crown? Have you done recent bleaching where your other teeth look whiter making the crown appear to be now darker than it used to? If you have dental insurance and want to make sure they will pay their portion they likely won't if the crown is less than five years old and you've been on that same insurance plan for the past five years.
You can always change crowns. If the crown is discolored over time you have to know why it happened. Was it a translucent crown and the underlying tooth color change?