If you rename a user and they log into Sametime, they will not see the persistent chat history. This is not a bug and the loss is not permanent. You need to run the database tool to synchronize the data properly. The Sametime 12 documentation describes everything. Here is a simple description of how to perform this process.
This is a companion discussion topic for the original entry at https://alichtenberg.cz/losing-the-persistent-chat-history-of-a-renamed-hcl-sametime-12-user-how-to-fix-it/