When 10 announced that its Adelaide bulletin would be segregated from Melbourne production, they did so in November 2022 - giving the audience in that market at least three months’ notice. Even so, the bulletin wasn’t fully local - with Kate Freebairn presenting from Sydney, as is still the case today.
If 10 does return Brisbane to being a separate bulletin, they would need to announce it with a similar period of notice, but as someone has noted, it would require an upgrade of its facilities.
And then there was this article which suggests it would only happen if the local Adelaide bulletin’s ratings improve - and from what I know, they haven’t (someone please correct me if I’m wrong). This suggests that Brisbane will remain stuck with the composite bulletin it shares with Sydney until such time when Adelaide’s ratings improve, which IMO does not make any sense.