A big thank you for the feedback you've provided on this feature request.
We've been working on this issue and have good news: from Crowd 2.7 (soon to be released), you'll be able to rename users (i.e. usernames) in Crowd's internal directories.
Crowd will also detect any renames that are made in a remote directory, including LDAP servers.
If you have any further feedback or improvement requests around this topic, please create a separate issue as we will be closing this one.
Thanks again for your continued support,
Atlassian Product Management