Depends on the boxing stance, not all boxing stances is upright. Slick style boxers would do well. If muay thai base mma fighters can adjust their stance for wrestlers, so can boxers. A lot of amateur boxers are already in mma and became UFC champions. That proves it's possible with just amateur boxing level. So there's already data to go on, and is isn't just speculation. If elite level boxers spent the same amount of cross training, they most likely would be champions in mma as well. A lot of boxers have wrestling/grappling backgrounds and also other martial arts striking backgrounds, they just focused on boxing because of the money and they were good enough to pursue a pro boxing career.