I'd typed this up in response to Andre's comment to a previous post, but thought it'd be better as an independent post:
To begin with, I concur in the praise of the appointment of Judge Harrison. I've always enjoyed working with Judge Harrison and his assistant county prosecutors in the past.
Canon 3E(1)(b) of the Code of Judicial Conduct would be applicable here. A plain reading of it would lean towards disqualification. Attorney General's Opinion 2006-00651 dealt with a similar situation. In that opinion, a former ADA had been elected circuit judge. The new judge wanted to preside over criminal cases in the 4 counties in which, as ADA, he did not handle cases. The AG's Opinion held that there was no absolute disqualification for the criminal cases in those 4 counties.
The difference here is that Judge Harrison was the lead county prosecutor in a single county judicial district. I think he's got to wait until new criminal cases start being indicted. In all honesty, that will probably only take a month or two.