If this incident is true, then I can see both sides of it. Never a good thing to change the ending without everyone knowing about it, but Brock could've handled it more maturely- after all, he's the "new guy" right now. If he's handling things like this after 1 month back, how will he react to something of a larger caliber that doesn't go his way after he's been back for a year?
One more note in Cena's defense: It's not like the "new" ending was that far off from the "planned" ending. The planned ending called for Cena to be carried out. The "new" ending had Cena selling his arm injury as legit and making it sound as though Brock has put him out of commission indefinitely with a tear or break. Same general effect. If anything, this new ending is fresher. We see guys "carried out" all the time (well, not all the time, but we see it happen). How often post-match do we get an interview in which a bloody and battered wrestler (with his lifeless arm dangling) says, "yeah, I think this guy just completely tore up my arm, I might be gone for a while."