An additional suggestion: have only good things to say about the instructor you are replacing, regardless of whether people are critical or praising in comparison. Be open to suggestions by the participants. Even as you are creating a new format, if there are things they liked before, I am sure there is a way of somehow incorporating it.
And – if you are good at this – try to learn the names of the participants. You will soon find that there are a few who set the tone and can make or break a change.