This seems like yet another example of a confused and possibly greedy salesperson. Every deal I have seen is swap what you have based on what it cost and apply that to the new product. Very occasionally, it will be more generous, i.e., where they will give an equivalent license even though one had not paid that much, e.g., moving from class based pricing. But, dollar for dollar trade in is the worst case. The only impact of swapping during a maintenance contract should be applying a partial year of old dollars (pounds!) to a full year of new.