There are two new rules for the 2023 NFL season, according to NFL operations. One of these changes is "Instant Replay Reversal" and the other is "Failed Fourth-Down Conversions".
Instant Replay Reversal
"New for 2023, when an instant replay decision results in a reversal under 2:00, the play clock will be reset to :40 instead of :25; unless another rule requires otherwise, such as when there is also a :10 runoff, in which case the play clock will be reset to :30.- NFL Operations
Additionally, inside 2:00, reversing from a ruling with a stopped clock to one with a running clock requires either a :10 second runoff or a charged team timeout."
If the officials look at something and decide to reverse the call, the play clock will be set to a full 40 seconds as opposed to 25 seconds. This is only under the 2:00 mark of a game.
Failed Fourth-Down Conversions
"All failed fourth-down conversions will now be an automatic booth review, similar to other turnover situations. Head coaches are prohibited from challenging a failed fourth-down conversion.- NFL Operations
However, successful fourth-down conversions still require a coach to challenge unless they occur inside the two-minute warning or during overtime."
This rule means that every time a team goes for it on fourth down and fails to pick up the first down, it'll be reviewed. Sometimes it'll be obvious but other times, such as on runs on 4th-and-Inches, this will be a welcomed review.
As the rule states, head coaches can't challenge a failed conversion on fourth down. Coaches can challenge successful fourth-down conversions, however.
These are the only two new rules for the 2023 season. We'll have to see how these work out this season.