Amy and Sheldon's Relationship Agreement has apparently been upgraded into a Marriage Contract.
Sheldon's friend Tam was first introduced in the second episode of Young Sheldon (2017), Rockets, Communists, and the Dewey Decimal System (2017), and he now crosses over into 'The Big Bang Theory' almost 11 months later.
The flashbacks to earlier episodes that now include Tam are: The Staircase Implementation (2010) (Leonard responds to Sheldon's ad for a new roommate); Pilot (2007) (Sheldon and Tam meet Penny for the first time); The Bath Item Gift Hypothesis (2008) (Sheldon hugging Penny for the first time); and The Einstein Approximation (2010) (Sheldon's shenanigans in a ball pit).
Title reference: a visit from Sheldon's childhood friend Tam upsets things.
Sheldon (Jim Parsons) mentions to Tam (Robert Wu) that his friends retrieved him from a dumpster during a "short-lived parkour phase." This would have been one of very few times Sheldon has participated in anything sports-like during the entire series.