who were Keaton and JK Simmons going to play

My guess is JK might've been George C Reilys character (bit similar to Terminator Genesis where he plays the older version of the young cop in the opening), Keaton might've been John Goodman character? Or SLJ?.. actually Keaton & JK could've been playing any of the 3 older characters (SJL, GCR, Goodman) as they both do comedic and bad ass roles regularly.. my guess is the makers of the film wanted Keaton and JK because they were 'hot' at the time with their Oscars when Kong will have been going into production (ok MK didn't win for Birdman but he's one of those that everyone thinks did as he should've instead of Redmayne) and they agreed but had to drop out for whatever reason and they had to get sort of equally famous actors

Edit: just checked wiki: 'On August 6, 2015, Deadline Hollywood reported that the studio was in early talks with Samuel L. Jackson to replace the role which Simmons vacated, while John C. Reilly was being eyed for Keaton's role, but not offered it yet.'