Billionaire Elon Musk is already floating major changes for Twitter as he begins his first week as owner of the social-media platform. One proposal would make some users pay if they want to keep a blue check mark on their profile. Twitter has historically used the mark to verify higher-profile accounts, including Musk’s, so that other users know it’s really them. Critics have derided the mark as an elite status symbol. Musk has invited a group of tech-world friends and investors to help guide the San Francisco-based company’s transformation, which is likely to include a shakeup of its staff.