If you've got a good quality aerial (together with good quality cabling), then you probably won't need to change it for digital reception.
If you find that you're not getting a strongth enough signal, you'll need to swap to something like this:
http://www.maplin.co.uk/Search.aspx?criteria=p t18u&doy=24m5&source=15
About two thirds of homes already get digital TV (mainly through Freeview boxes) but the streets don't seem to be full of aerial fitters changing people's aerials. So it would seem that most people find their existing aerials work OK.
Chris