This isn’t exactly a secret, but it’s not well socialized either. Thanks initially to the 737-MAX grounding and now due to COVID waivers, periodically Southwest Airlines lets you change to any flight ± 30 days when you book within certain windows. (I’m not sure they want to do this, but they run an archaic computer system and it’s what they can do without breaking it.)

The play is to find the cheapest award or cash booking on a ticket you want to fly, book it, then change to the actual itinerary you want within 30 days for no additional points or cash. (Book SLC-SAN at 4:30 AM on a Saturday for 4,008 points, then change it to Monday at 5:00 PM.) You can do this online, no need to talk to an agent.

Currently, the booking window for free ± 30 days changes is February 11 through March 5, 2021, so you can fly cheap between January 11 and April 7. Yes, Southwest flies to Florida too, sorry.

Vintage NEC computer from the 1960s with relays, knobs, a printer, and tape spools.
Southwest Airlines’ Amadeus Computer Reservation System (circa 2020).