If you know anyone with a set top Freeview box, ask them if you can borrow it and hook it up to your telly. If you can get the existing freeview channels you won't need to change your aerial.
I suspect that a lot of people are being told that they must shell out for a new aerial when it's not necessary. I have two elderly neighbours who have been conned like this in recent weeks. I have a 15 year old aerial from which I run 3 TVs, each with their own STB and apart from an aerial booster I have nothing fancy at all.