I'm hoping I'll be able to just go back to my old username, will probably pick something similar if I can't.
There is a method I use to manage my notifications, by archiving them in a searchable file. I mostly use it to find old conversations, sometimes I've used it in the past to research for tribute memes if a user deleted their account. When you archive notifications, it's like a snapshot where usernames are preserved in the file. I archive them all two or three times a year.
I keep my archive files on my PC and only use them there if I need to. There is probably a good way to do the same thing on mobile but I keep hitting bumps and haven't worked out all the kinks.
It takes a while to do this and one wrong click would take you back to the start of the process. Clicking the "Show More" button will load the next 20 notifications to the page. The Notifications page is not like your My Comments or My Upvotes pages with a "Next" button that loads a whole new page.
You can keep clicking "Show More" and load all your notifications on a single webpage. For me, that takes a while, about 15 minutes on a PC, about 25 minutes on my mobile device. Here's the rub, one errant click and you will be back to square one. When you have all your notifications loaded, you will see the very first Level Up notification saying "You've earned 10 points! New icon available!"
On a PC, right click in Chrome, click "Save as" you want to navigate to a folder and save Webpage, Complete. Since I do this a couple times a year, I have the date as my folder name.
On mobile, there is an option to download the page. It's a pretty big file for me, about 70MB. For me I don't have a method down to open the archive only when I want, or open the live notifications only when I want.
Once you have a file saved, you can open it with a browser, use "Find" or Ctrl-F to search the file. You can search by username or words in the first 140 characters or so that are visible in the notification.