This is unclear. If you booked before May 28, the expiration clock starts when you cancel post May 27. WGA fare rules apply unless you actually booked Basic. Basic can't be booked until May 28. Unless I misunderstood something.
I hope Southwest clears this up soon. You get free bags after May 27, but that's only because the flight was not changed.
But if the flight is changed in any way after May 27 including cancellation, the WGA flight is instantly converted to Basic and then cancelled is how I'm interpreting it. August trips booked in March are going to be bad for me with rapidly expiring credits. Flights booked today April 15 will theoretically expire October 15 under the most unfavorable interpretation. I have a bunch of flights I will need to switch to WGA+ if it's the worst interpretation because I need free SDC.