Finally got one of those unique $20 flight change offers. Anyone know how it actually works?

Southwest finally sent me one of those $20 change offers I always see people mention. Not a lot, but still lunch money.

If I accept one of the suggested changes, do I get to keep the $20 credit even if I later cancel the flight and don’t fly? Or is the credit only added after I actually take the flight?

I was probably going to cancel this trip anyway, so just trying to figure out if it’s worth messing with or not. Anyone done this before?