Advice needed on whether to refinance HELOC only, since first mortgage is halfway paid off.

We refinanced our first mortgage in 2005 at 5.5% for 15 years; it is now at $190,000 with 7.5 years left. In 2007, we took out a HELOC in order to add on to our house. We owe $215,000 on the HELOC. We are currently in an interest-only repayment period (although we always add more to the required payment in order to decrease the principal), and our interest rate is Prime + 0. Our credit score is around 760. If we refinance our first mortgage, the interest we will pay over 15 years will be more than if we keep our current mortgage (since we have only 7.5 years left on it), despite the lower rate. We plan to stay in our home permanently; it is in a highly sought-after area of our city of older (80-100-years old), well-maintained houses. We would appreciate any advice--thanks.