Hi Sleepy Tom,
Yes, switching loans to a lower rate is common. Just make sure the saving you will make is worth it as you will no doubt have to pay a 'settlement fee' to get out of the loan.
Some companies offer a rebate on the interest - but it normally comes with heavy conditions. They normally require you to pay the loan off over the full term - rather than settle early, something which you are looking to do with your previous loan. Be careful of this as most loans will never go full term and you will have to pay the full interest.
Tim