how much change that involves depends heavily on how smooth the original translation is

Is why translators should be writers too. Not that that necessarily helps. I can produce something that sounds good to my ear and also renders the Japanese meaning, but I have to put it away for a week and then look at it again before I can actually turn it into the best English there is. My translations cause me acute pain afterwards, precisely because I don't usually get to do that when working to even a self-imposed deadline. Too much to expect that pro translators have that kind of temporal leeway either.