Former The Cab guitarist Crawford replaces Ryan Ross, while The Brobecks bassist Weekes takes over from Jon Walker.
Ross and Walker split from the band earlier this month to form new group The Young Veins.
Speaking to MTV, drummer Spencer Smith said: "[Ian] is probably the most talented guitar player that me and Brendon [Urie, singer] have ever played with, and Dallon is a really nice guy and a really great bassist."
Panic At The Disco will fulfil all their summer dates, which include support slots with No Doubt and the reformed Blink 182. They are also working on the follow-up their Pretty.Odd LP.