Despite the fact he's been in his camp from day one - AJ has officially confirmed Rob McCracken as his head coach. http://www.skysports.com/joshua/news/34644/10678779/1
That's good news for Joshua. Never been a big fan of Tony Sims and, although as you rightly point out, McCracken has been there from the get-go, a single voice rather than two will be beneficial. This is more to do with Sims being elbowed out than McCracken stepping up.
He's always been his coach as far as I'm aware but it couldn't be officially announced due to contractual issues with Team GB. Sims has only ever really been his corner man with McCracken never far away in fights.