If you've had a good payment history in your existing mortgage then you may be able to qualify for a better rate if you had poor credit in the past. Lenders can see that you're a good risk, and that makes the qualifying process easier.
Your lender will need to verify your income, employment, account balances, etc, just as when you first financed your home. Also you will need to provide current paystubs, bank statments, 2 years income taxes, and order a new appraisals as well as pull a new credit report.