So, the actual reason for the change is likely their dumb handling of capitalization in the original scheme. To use your username as an example:

copygirl#1234 Copygirl#1234 COPYGIRL#1234 CopyGirl#1234 Etc etc

Every permutation of capitalization is available as a distinct username. And with the low price of Nitro you get to customize your discriminator, which could make impersonation a very real problem.