Keep an eye on December 30 — that's the date after which candidates can't remove their names from the party primary ballots. And it's important because it comes before the filing deadline for those contests.
Candidates can always bail if they're incapacitated or if they move out of the district (or the state) they sought to represent. But they can't voluntarily back out of a race after that date.
That'll stifle efforts by incumbents to hand pick their successors by backing out at the last minute and letting the friendly newcomer sign up before any other rivals have time to react. An incumbent can simply wait until the last hour of the last day to pull out, let their political heir know to sign up at the same time, and others find out too late to get into the races. They'll only be able to pull that this year if they wait until the last day to sign up — a move that could embolden challengers to pounce.