Changing to a 30 year mortgage will slow down the payment of principal considerably, due to the amortization of the payments (i.e. for the first couple of years you will be paying primarily interest and relatively little principal). However, changing it from a 20 to a 10 or 15 year mortgage (with potentially the same monthly payments due to the lower rate) will cause you to pay off the principal much faster, thus allowing you to take out another mortgage or line of credit when you think you’ll need it down the road.
Don’t take this as advice. Just pushing in a thought there for you to think about…