Amending Virgin booking?

[legolass]

We used Alamo this year and loved going straight to the garage to pick the car. So next year we have booked flights to Miami but the cost of Alamo is so high! However, after reading on the dibb I have a price with Virgin Atlantic for £412 which I think I will go ahead and book (Dollar is around £270 ... but I think it's worth the extra?). But we haven't quite decided if we will stay at Miami airport overnight when we arrive then pick up the hire car and drive up to Orlando the next morning or if we will pick up the hire car immediately and drive even part of the way. Once a booking is made with VA, can we amend the booking to add another day (or remove a day?) or do we need to make a decision before we book?