Sense8
This short-lived Netflix show still had time to get a recasting squeezed in. Aml Ameen (known on the show as Capheus) was replaced with Toby Onwumere during season 2. It was apparently due to "some sort of conflict between Aml and Sense8 co-creator/executive producer/director Lana Wachowski that sparked up at the season 2 table read in Berlin and worsened as filming continued in India." Sounds dramatic...
Furious 7
Are you ready for a tear-jerker? Paul Walker had passed away before the film was done shooting. With some careful CGI work and clever editing, the cast and crew were able to recreate the deceased star on the screen. His brothers were also used for some behind shots and other movie magic scenes.