Fuller is still involved, but I think everyone came to the realisation he just can't run the day-to-day stuff on the show. Fuller has set the foundations for the show and he is clearly passionate about what direction it was going to take - maybe a little too passionate.
I have a feeling the reason why the entire cast except the lead has had their positioned filled is because Fuller was being too much of a perfectionist and delaying production. I mean, this is nothing new for trek... GR HATED the idea of Patrick Stewart as captain and Janeway's role was recast at the last minute due to creative differences between the actress and the showrunners.
Of course, there are reasons to be concerned... But don't lose your heads here people. Trek having wavouring show runners is like a tradition.
