Q: Is it OK to change the navigation of a website once users have logged in?
Yes, if the user experience is completely different for logged-in users vs. users who may just be browsing for company or product information. One good example of when and how to do this is banking and finance sites.
Often, the general user experience includes a large range of products and services organized by category (for example, business solutions, personal finance solutions, commercial solutions, etc.), while the logged-in user experience focuses on a smaller subset of functionality (checking account balances, setting up alerts, and making transactions such as transfers). Keeping the navigation scheme the same for these two very different audiences would cause great inefficiencies in the tasks of both.
If you are changing the navigation for the logged-in user experience, it should be extremely obvious to the users that you are doing so. Don’t be subtle here! For example, don’t keep 4 out of 7 navigation categories the same; consider changing the style and templates so that while you retain branding and visual design guidelines, users will know they are engaging in a completely different experience.
Also, make how to log out obvious so that logged-in users can easily access the general view if they want to.