sdsearch

They determine that a flight is oversold at boarding time, because the whole reason some airlines overbook some flights is because of having statistics about a certain percentage of people usually not showing up at the gate. That's why can ask for volunteers, because the possible volunteers are in the gate area, and the overbooked status was not determined until there were passengers in the gate area.

But your case is about Southwest moving you to a different flight about a week before departure. So that is not a situation you can easily compare to what is done for overbooked flights.

This is an unusual situation, so it's hard to compare. United has a different policy, in that they don't limit their passenger load to a specific percentage, rather they notify you in case your flight goes over 70% load and in that case they give you free change possibilities. But since Southwest is the only major airline with open seating, so their current Covid-19 "seating" policy is different than any other airlines' policies (where they have assigned seating and can simply block certain specific seats from being purchased), and so your situation cannot be compared to the policies at other airlines (which, among other things, have the option of moving some passengers up to a higher cabin if coach is the only cabin oversold while cabins up front are undersold).