Not really sure I understand the question.
But generally what I think people are suggestion is that you create a panel to display the detail information of a client. For example maybe you have data like firstName, lastName, address etc. Then when you prompt for a client you get the client number and then just refresh the data on the panel. So you do something like:
So there is no need to recreate a panel just because you change the client you want to display information for.
Maybe there are times when you have data in a JList or JTable and you need to change the data. Again, there is no need to recreate the panel of the component. Instead you change the model of the component:
So there is no need to remove or create new components.
The other part of your question deals with the CardLayout. A CardLayout is used to display different panels in any order. So again the panels are created with components at design time. Then at run time you just populate the data as required. The Swing tutorial on
How to Use Card Layout shows how to initially create panels and then swap tem as required.
There is no need for methods like removeAll().
Of course there are times when you need to dynamically create components so you have to look at each situation and decide which is the best approach to use.