Cloud Kicks (CK) tracks the support level of its customers on the account record page.

Add a rich text area to the Case Lighting page > Set the component visibility of the rich text area to show when the account support level is platinum.

Create a text-only Visualforce page > Drag the Visualforce component into the Case page layout > Set its visibility to show when the account support level is platinum.

Create a text-only Visualforce page > Clone the case page layout > Drag the Visualforce component into the page, and assign the layout to platinum cases.

Clone the Case Lightning page > Add a rich text area to the new page, and assign this page to platinum accounts.