The east and west stands were built upon the old stadium, and then they added the North and South endzone stands. I think originally, the idea was to build a 2 tiered lower and upper desk on the east side of the stadium, and maybe on the west, I can't remember. I know for sure it had the original idea of the East. I think that got scrapped and they stuck with just adding on to the East and West stands already there, and then they built the endzone stands.
So really only the East and West need to be knocked down and rebuilt. I personally don't think they need the "fill in" the corners with seats... maybe some premium seating like structure that is gaining popularity in stadiums right now, that have a table in front of them, larger leg room, heck they could even make these section heated for when it gets cold. I still think that added party decks that connect all the sections in the corners would be awesome. Kind of like Tennessee is doing
Add something like that at each corner would be great. Also Arkansas has party decks in each endzone.
But BYU would have to start with knocking down the East and West Stands. If BYU built a facade structure around the stadium, they could add some cool decks around the North and South endzones.