Have you ever had to change the substrate in an established tank?

A lot of the time when you are adding the substrate to your new marine tank, you can think of using a specific brand or grain size and then once the tank is fully established, it might not look or be what you are looking for. Other times, you might need to change it in order to have the substrate act as a better buffer when it comes to water parameter issues. Although it might not be ideal, how often have you come across this issue with your own tanks, and do you end up actually changing the substrate around, or do you just add onto the current substrate with whatever you wanted that might be more ideal?