Don't bad mouth anyone! Sometimes tricky to talk around when discussing why you are leaving current job - but rehearse and be ready with a positive response. Just keep it all about your desire to keep growing - you've become quite competent in residential, and while you have experience in commercial, you'd like to spend more time honing your commercial savvy...

Something like that. Just believe in what you are saying and you will sound honest. It's much harder to lie and it usually shows...