When to replace 17 year old dental crowns? (photos)

After an accident where i needed a root canal i got all porcelain crowns on my two front teeth. This is 17 years ago. Dentists have always said they look nice. My new dentist wants to replace them since she says the colour is off. She wants to replace them with cerec crowns. I am reluctant because i heard they are not as nice and also because i feel pressured. What are your opinions