A Pac-12 spokesman confirmed Friday that the senior, who missed the 2011 season with an undisclosed medical condition, would be forced to sit out one year to meet conference transfer guidelines.
Armstead graduated with his degree in sociology in December, which would allow him to use an NCAA rule that grants immediate eligibility to play while enrolled in a graduate program not offered at USC.
However, that does not supercede conference rules, preventing Armond from joining Arik at either Cal or Oregon for one season.
Armstead told the Orange County Register last week that he is considering Auburn, Arkansas and Notre Dame as potential transfer destinations.
The Tigers and Irish are among the five finalists for Arik. The two brothers, along with their father Guss, are visiting Auburn this weekend.
Armstead, who started 17 games at USC playing end and tackle and amassed 59 tackles, 10.5 tackles for loss and two sacks, could also declare for the NFL draft before next Wednesday's deadline.
Armstead was hospitalized in March with chest pains and was held out of spring practice. USC doctors allowed him to resume conditioning drills in August, but head coach Lane Kiffin said in September that the 6-foot-5, 285-pound lineman would not play and instead redshirt.
Aside from ruling out a heart condition, neither Kiffin nor the Armstead family have commented publicly on the exact nature of the condition.
Armstead's decision could also play a role in where Arik, a five-star recruit rated as the No. 2 player by Scout.com in the Class of 2012, decides to play.
The younger Armstead originally committed to USC last June before decommitting in October, citing the school's unwillingness to clear Armond medically as a factor.
CBS Sports first reported Friday that Arik is enrolled at Cal, which is necessary for him to participate in spring practice. But until he attends class, Armstead is not bound to the Bears and can still be recruited.