The username for the launcher is remembered using a registry key (which I cannot look up and post at the moment, because I'm not at home). I'm not sure if this will also change the ingame login name.
In your STO directory (or is it the userdata directory I'll look this up when I'm at home) there is somewhere a text/configuration file where the account name is stored. Maybe this is the one used to remember the ingame login name.
If it's neither the registry tweak nor the configfile it might be a bug in the linked Cryptic-PWE-Account-Management.
As I said, I'll lookup the above mentioned when I'm at home and post it here.
I'm having the same problem. I've tried searching every file in the STO directory, as well as the entire Windows registry, for my old username. No dice.
EDIT: I also looked for a per-user and system-wide AppData directory (under C:\Users\username and C:\ProgramData), and I couldn't find and STO data there at all.
EDIT 2: Gaaaah! Further frustration: When I load GameClient.exe directly, rather than running Star Trek Online.exe, it shows my correct, new username. When I excitedly reloaded via the STO Launcher, it reverted back to the old name. Perhaps this is an issue on Cryptic's servers, where it sends you the username at some point during the login process. This seems to be an edge case for those of us who converted from Cryptic accounts to PWE accounts.
Launcher successfully remembers my perfect world login since the game forced me to create one, however when I log out of one character to another the saved login is my old one.
Nothing I have tried will change it
Did you ever find a solution to this? I have the same problem having been forced to change to a perfect world account and no longer being able to have the same username (for reasons best known to Perfect World/Cryptic).
The problem is on PWE's end. Their database server is the problem. I first reported this to them over 6 months ago. No fix yet. In fact no response yet. Seems like in 6 months someone could fix a database error. Oh well guess not. BTW there are several threads about this problem. None have a solution or reply from PWE or Cryptic. Would be nice.
I suspect this is the same problem screwing up my Neverwinter access, since I seem to have one "Display name" for STO and Champs, and one for Neverwinter. I couldn't find a reg entry at the location given above for it, and adding the pref login line to my launcher .prefs didn't help either. Humm.