Purchasing a brand new caravan

Hi, we are new to touring and picked up a second hand van around 8 months ago.  We have loved every minute of it and decided at the show at the NEC in October to purchase a new van from a dealer in South Cave.  We are picking the van up this Saturday and i am getting nervous that they are going to change the part exchange price they have given us (without seeing it) for our old van.  Are we within our rights to walk away if they change the part x offer?  Please feel free to offer any other advice regarding picking up a new van. Thanks