There have been reports that people who bought EBCI and later rebooked their flights ended up with worse boarding positions than they would have had otherwise. For example, two couples buy Wanna Get Away fares plus EBCI the same day. Months later, Couple #1 rebooks at a lower fare. Couple #2 does not rebook. When boarding positions are assigned, Couple #2 has low A boarding positions, Couple #1 (who rebooked) are at the back of the A line. That suggests that they purchase date was reset when they rebooked their flight.
Fare category also plays a role. According to Southwest's website, those who purchase Anytime fares get better boarding positions than those who purchase Wanna Get Away fares. It's possible Ding fares are treated separately from WGA fares so if you rebook from WGA to Ding, it might put you further back in the boarding queue.
Ignoring Business Select fares (which get assigned to the A1 to A15 group) I imagine what happens is that at 36 hours out, the computer generates a list of passengers ordered first by Status (A-Lister or not) and then by fare class (Anytime fares board before Wanna Get Away fares). Within each class the list is ordered by EBCI purchase date. I think what happens is if you rebook, your EBCI purchase date gets reset so you end up further back in the line.
Once the list is generated, boarding positions are assigned based on your position in the list. At 24 hours out, non-EBCI passengers start checking in. Boarding positions are assigned starting with the lowest open position in the boarding queue.
If any passenger who has checked in (automatically at 36 hours out or manually at 24 hours or less out) cancels their flight after getting assigned a boarding position, that creates an opening that will be given to the next person to check in manually. That's why some passengers can get a very low boarding position even though they bought a ticket that day or checked in just a few hours before their flight.