https://tvline.com/2018/11/14/the-big-bang-theory-iain-armitage-season-12-young-sheldon-cast/

Iain Armitage and his Young Sheldon dad Lance Barber and brother Montana Jordan are crossing over to the comedy their show was spun off from. CBS won't reveal how the trio will appear on The Big Bang Theory, but it'll likely be via flashback.

Young Sheldon to appear on The Big Bang Theory via VHS videotape (https://tvline.com/2018/11/26/young-sheldon-on-the-big-bang-theory-season-15-iain-armitage-season-12/)

CBS has revealed details of the Big Bang-Young Sheldon crossover episode, featuring guest appearances from Iain Armitage, Lance Barber and Montana Jordan. The Dec. 6 Big Bang Theory episode, titled “The VCR Illumination," has the following description: “Sheldon and Amy are still down about their theory being disproven, but a VHS tape from Sheldon’s past inspires him not to give up.”

That sounds bad. Good thing they’re wrapping TBBT up.