Changing the ID was a workaround for Joomla! 1.0, 1.5, 1.6 and 1.7. Randomized IDs have been a thing since Joomla! 2.5. Moreover, changing the ID would cause existing items (e.g. articles) to be "orphaned", causing other subtle issues. That's why it was removed.
The recommended way to get rid of old-style Super Users with an ID of 62 / 42 is to create a new Super User account. Log in to your site using the new account. Now edit the old account and change its group assignments to contain only the Guest group (effectively denying login for that user). After saving that change you can do anything else you want, e.g. change the username and block the user.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!