Thanks to Jean-Edouard Babin for reporting it.
How to reproduce:
1) connect to bip with an irc client using username (eg 'Pilou')
2) disconnect irc client
3) in client configuration, change the case of username (eg 'pilou')
4) reconnect to bip -> bip log FATAL message and exit
If old nick and new nick are equal ignoring the case of the characters,
don't delete store associated with old nick: only rename it.