It was unfortunate for the BBC that the changeover coincided with a couple of especially poorly recorded programmes (Garrow's Law in particular was appalling - not for lack of sharpness but for some very intrusive artefacting in porrly lit scenes) and a few technical issues with the new encoders.
Interestingly, season two was shot on Red cameras whereas the first was on Sony F35s.
Also they purpose built the prison sets for season two. Season one's were shot on location.
It was so much better.