You summed it up in the title of this post lol. Jansen got fired and Dallas switched to Chad for prep. Doesn't take a genius to figure out that staying with Jansen for prep would be a bit embarrassing for Redcon1 and awkward for both Dallas and Matt. Redcon1 wouldn't be able to use any training footage if Matt and Dallas trained together, they would only be able to film Dallas alone eating his meals and talking about his program avoiding mentioning Matt. Switching to Chad will allow Redcon1 access to Chad and perhaps even some stories from the past. Good move on Redcon's and Dallas' part.