I'm pretty sure you'll need to replace the files but maybe try it without replacing first. I didn't need to rerun any server behaviors when I upgraded, I just replaced the files. You may need to make other changes as it really depends on which extensions you're using, or if there are any manual customisations. I don't know if you have the option of testing the site in advance by cloning it to a testing domain and running it on php8 - that's what I did so that I could find and fix any errors before upgrading the live site. You can also use PHP error reporting to help diagnose and fix any errors - if a page doesn't work, add the following code to the very top and load it again:
<?php
error_reporting(E_ALL);
ini_set('display_errors','on');
?>
Take a note of the error and either ask in the forums or you could also trying asking ChatGPT as it's very good at working out what might be causing an error and how to fix it: https://chatgpt.com/
