If the rate doesn't jump when you modify, you should also be able to just extend the current reservation and have the free days applied via phone agent after return. That will save you a trip to the airport
if it's an airport location, be careful and see if you can not pull into the return lane, sometimes there would be 20 cars in front of you and you will be blocked in. Most locations are pretty good about leaving lanes in between returned cars open and leaving space between vehicles, but I have seen cases where the gap between cars are so small/they used all lanes and you won't be able to get out
PS. Sometimes calling/emailing the location also works, but YMMV