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In the old student section 13127 seats were lower bowl compared to 6878 seats now.89.9 % vs 58.2%
The old student section also had more seats at 14595 total seats vs 11813 now. 19.1% reduction in student seats
I can see why a BYU student that didn't have a guarantee that they would be in one of the lower bowl sections that they might choose to not go to the game. Much like basketball, moving the students farther from the field can have some benefits in consolidating the fans, but there is also a risk that students don't want to sit high up in the corner of the endzone.
(I calculated the seats based on the BYU season ticket interactive map.)
For the old student section I used this map and estimated the number of rows in section 132 and 136