I've set a user to a higher rank than default. When he changes his name, his higher rank resets to default rank.
If the user changes the name while he is lower than default then he will keep his restricted rank.

We have tested the above a couple times.

Also at some point his username began to get duplicate names on my end when I received private messages from him:
Name in user list: CustomUsername
Name in PM Title: CustomUsername(3)

I am the only owner of the channel in questions, no moderators or managers.
Tixati version: 2.88 (both)

Edit: I now see him 3 times in the user list as "online" under different IDs. His current and latest nickname has yet another ID but he's really only and reachable to PMs. These older names are zombies, appear online but PMs are not acknowledged.