I've never had a mortgage but if it is like a car loan...
...you can always pay more than the minimum payment in order to complete the contract earlier. I can't assert this is true for mortgages, but it seems like it should be within one's rights to pay off a loan whenever one wants to and one has the funds.
Aside from the car-loan analogy, a lower rate does not only reduce your payment. It also reduces the total amount you will have paid by the end of the contract. In short, it makes you money.