*** An Announcement is not Dissolution ***
Under section 23(1) of the Constitution Act, the Lieutenant Governor dissolves the Assembly on the Premier's advice, by proclamation, so the press conference confirming it is not the act. The act is the proclamation.
At that moment members cease to be MLAs and party standings freeze, which means a change in affiliation only counts if it's recorded before dissolution.
Interestingly, that dissolution is recorded by date, not to the minute.
And there's no published rule for how a member records a mid-term party change either. It's parliamentary practice, handled by the Speaker and Clerk, so only those offices can say whether a notice met the requirement.