OK, so I believe there's a way to book this on the site despite it not showing up in search or as a popup to buy up to main like for other airlines. I have not purchased yet using this method but the pricing works out as expected so I'd like my odds of everything working out without a hitch.

On the screen after you've selected both flights, it should be showing you the flights, booking class, and prices. Take a look at the URL, it should look something like this: https://www.visadiscountair.com/cnb/...7400910032SAS0 10 Saver 168.6,AS097811MAR19LASSFO06100745073HAS0 10 Saver 168.6),numPax:%2720000%27),...)

Basically it is listing the flight numbers/airports/etc in the URL. I have 2 flights listed in the URL above but if you have connections, it'll have more. You'll see the word Saver in the URL in a section for each flight, and to the left of it the number 10. Change 10 to 11 for each flight you want main fares for. At this point, hit enter and the page will load again with Main fares listed. Should raise the price by $30 per passenger per flight for most destinations (or whatever the right buy-ups are for your particular itinerary) if you're upgrading both ways. It'll also show an orange box warning that the price increased, which is fine, because it did go up as expected. Proceed through the purchase from there.