After careful inspection of the trailer for JM, only one notable difference stood out to me: the model shot of the Beijing NAS protest march. Compare:
From the (HD transfer of the) film:
Versus the (HD transfer of the) trailer:
It looks like every building has been tweaked, with added details, signage and colors. I suspect it was changed late in the filmmaking process, as every other shot in the trailer appears to be final; ie, no low-res, prototype CGI, for example.