When a user modifies their username, all hyperlinks that they have distributed become, thereafter, inaccurately attributed to whomever else decides to utilise that username. Even if that does not occur, it means that all telemetry provided by that argument is rendered useless, for it no longer references the actual referee.
An example is https://meta.discourse.org/t/70952/2?u=roke_julian_lockhart, [1] which should be https://meta.discourse.org/t/70952/2?u=129529, for it to remain active across username replacements.