I don't think that technique works too well. The bigtable dataset (and I believe firebase as well) both keep the original comment and original username. If you change your username, then someone can trivially check old comments on bigtable and see how HN currently renders those same comments today to learn the new name.

There may be a good question about if those firebase and bigtable datasets are stored in compliance with GDPR, I suppose one would need to send delete requests there as well? Although I'm not sure about contact info. It's quite difficult to delete things on the internet...