yeah this has been brought up before! unfortunately, it'd be extremely difficult to implement such a system, given the site's extensive history that it needs to cover (been around 21 years!) and also potentials for abuse and whatnot.
i WOULD like such a system, and if one was available back in 2016 or so i wouldn't have had to make a new account (likewise with when i rejoined in 2021), but the reality is it'd be a very complicated thing to implement. it'd require under the hood changes on how accounts are handled, and depending on which solution you go for it can either result in dead links, be ripe for abuse from the wrong ppl, or just be a pain to implement.
retro does bring up a good point: you've just recently joined, you level up fast on the site for the early levels, so you can just make a new account with the desired username. just keep in mind you can't change it so choose carefully!