lynn117, a new crown is preferable because having the crown drilled alters the strength, even after it's filled.
Merlina, my own dentist replaced a few crowns that I ended up needing root canals under, free of charge. But he told me that not all dentists do that. You'd have to speak with your dentist about that; he may or may not agree with the retreatment; if he does, he may replace it for free or a discount but that is the dentist's choice. Basically, they fall back on the "you can't predict what a tooth will do" excuse, which is somewhat true. Sometimes, a root canal fails, even if it's done properly.
i had a root canal and they drilled thru the top of the crown, did the rc, and put a filling in it. its not as pretty but its not a tooth that shows so who cares. so why must you have a new crown?