Jim Davis passed away suddenly during production of the fourth season. The production crew had his character, Jock Ewing, go offscreen for a year while they figured out how to write out his character, then finally perish in an offscreen plane crash.
Barbara Bel Geddes' character, Miss Ellie, does not appear in the rebooted series, as the actress had passed away in 2005. She is stated to have died of a heart attack sometime between the end of the original series and the pilot of the rebooted series.
J.R. Ewing in season two of the rebooted series, in the wake of Larry Hagman's death.
The Danza: Harris' mother Judith Ryland is played by Judith Light, while her granddaughter Emma is played by Emma Bell.
Fan Nickname: Not fans, but the actors themselves. When they looked and saw the actors playing the "New Generation" of Ewings (Christopher, Elena, Rebecca, and John Ross) were named Jesse, Jordana, Julie and Josh, they occasionally call themselves varying versions of "The J Squad" or "The J Team" .
Among other notable character actors, the producers of the 2012 series seem hellbent on featuring every supporting cast member of the FOX series 24 they can find, including Carlos Bernard (Tony Almeida) as Vicente Cano, Glenn Morshower (Aaron Pierce) as Lou Rosen, Faran Tahir (Tomas Sherek) as Frank Ashkani and Annie Wersching (Renee Walker) as Alison Jones.
They also seem hellbent on featuring Desperate Housewives alumni in their cast - Mary Alice, John the Gardener, Aidan the Bad Boy...
Old Shame: Victoria Principal refused to have anything to do with the revival of the series. According to Patrick Duffy, this is because of the fact that she doesn't want the show to interfere with her current occupation running a cosmetic company.
Everyone involved has been fairly upfront that Victoria was never really friends with the rest of the cast and wasn't really part of the inner circle of Larry Hagman, Patrick Duffy, Linda Gray and Ken Kercheval. The creator of the revival has said fairly explicitly that she doesn't even really want Victoria back and the rest of the cast don't seem to worked up about it either.
Miss Ellie (Barbara Bel Geddes was replaced by Donna Reed, who was herself replaced by the returning Barbara Bel Geddes.)
As a series regular, Jenna Wade was played by Priscilla Presley from 1983 to 1988, but she was actually the third actress to be cast as Jenna. The character first appeared in an one off episode in 1978 (played by Morgan Fairchild) and appeared in two 1980 episodes (played by Francine Tacker.)
Digger Barnes was first played by David Wayne, then Keenan Wynn.
Kristen Shepard was originally portrayed by Colleen Camp before the more well-known Mary Crosby took the role over.
David Ackroyd (and not Ted Shackelford) was the original Gary Ewing.
Reality Subtext: The death of Jim Davis (Jock Ewing). It was dealt with by putting Jock on various tasks for over a year before instituting a Bus Crash.
Larry Hagman died from throat cancer during production of the second season. The production crew worked around this by "jury-rigging" scenes to feature J.R. speaking to other characters (cobbled together from spare footage and voiceovers). While they had originally planned to keep J.R. around for years even in the event of Hagman's death (as an offscreen figure doing other tasks separate from the main cast), his sudden death forced the crew to rework several shots Hagman filmed as a final conversation with John Ross in "Furious and Fast".