You can change it same day as departure. Some airlines make it easier than others. Over on UA I've done OLCI not gotten pre, added KTN, rechecked in and got pre. AA, i can edit traveler info and add middle name and KTN. WN, remove and reapply rapid rewards number and check in again.

On a recent reward trip on Alaskan booked through Delta, it had my Delta FF but not my TSA info. I added it the day of my outbound and I was good to go.

It is impossible for US to change our own secure flight data within 72 hours of departure using the DL website. A DL agent can easily fix secure flight data any time. That (updated) secure flight data is then used on the next check-in transaction.

It's instant. Bought a last minute ticket for same day departure and Pre was issued on the boarding pass. True you can't add it if you forgot to when making res due to 72 hr rule.

A couple of things to clear up confusion.

At 72 hours pre-departure the airlines send all the Secure Flight Data to the TSA, who then come back with responses for each passenger. After that the airlines also send info on any changes/adds to Secure Flight Data at 48 and 24 hours before departure. The airline gets an electronic message indicating if a passenger is being given PreCheck for that flight during these sends.

When you first check-in the PreCheck has already been determined by TSA and the status sent to the airline. However if you don't get PreCheck you can have the airline check you out, and try checking in again. This works because the process of un-doing the checkin causes the system to re-send the information because it thinks something has changed, even though nothing really has. The only truly valid reason for un-doing a checkin is that something is changing, so the system assumes something has changed. Thus when you check in again, the answer may have changed from no PreCheck to yes for PreCheck because of the resend.