If you change your course selections in the Settings sheet after having already set up the spreadsheet, the Update Course Selections warning message will appear.
Khan2GC will need to create or delete course sheets depending on your new selections. The process can take up to five minutes. If you choose Yes, it will commence updating the sheet. If you choose No, it will revert your course selections to what they were before you changed them.
If your spreadsheet should freeze and you receive a message from your browser saying that the page is unresponsive, refresh the page. Even if you refresh the page, the update process will continue to work in the background. Note that after refreshing, it still may take several minutes for the spreadsheet to load and/or become responsive again. When it does, wait until your spreadsheet is responsive again before continuing.