Resolve user name change function for Wordpress-based social media network

Hello, freelancers! I have a social media site, wrought from pure PHP, that uses a forum in which members post threads and comments. Members have profiles and avatars, as per usual. The problem I'm running into is that when I change a username, the username change is not applied site-wide to the front end. In some areas, the old username still displays. I would like to solve this issue. I am currently using a simple plugin to change usernames when such a request is made, but it's not entirely effective. _ _ _ _ _ _ _ _ _ WARNINGS 1. Please note that I DO NOT ALLOW FREELANCERS ACCESS TO THE SITE WITHOUT ACCEPTING THE JOB FIRST. If you're bold enough to place a bid on a project, you should be confident enough to accept the offer and begin working. Asking for access without accepting the job will result in immediate dismissal. I am willing to discuss and negotiate allowed time for completion of the project. If the project looks too complex, I will understand the honesty. Try to be up front about your skills and don't waste my time or yours. 2. I am not inexperienced to managing these projects, to site administration, and some aspects of coding and design. In other words, I know roughly how much time a particular task should take a full stack coder familiar with CMS, PHP, and MySQL databases. Those who purposefully overestimate time on a simple task will be rejected, removed from consideration of future projects, and reported to Freelancer. Don't try it! Honesty and professionalism will get you much farther with me. 3. Please also note that this is an ADULT website, meaning it contains content that is adult in nature. If you have issues with adult sites, please save your soul and DO NOT bid on this job. Thank you!