Should I Refinance to Get a Lower Rate for a Longer Term?

refinance

I have had credit issues in the past, but have corrected by finances. I could get a 2% lower rate by going back from a 20 year mortgage, with 18.5 years left, to a 30 yr fixed. My payment now is $850/month. I have been offered a new refi making my new payment $600. If I were to do this and keep paying $850 (paying down principal), is there any penalty?