As VHG says, an existing TV aerial might be good enough to receive Freeview. However, digital TV is transmitted on different frequencies to the analogue stations, so the aerial might not be as well 'matched' to the relevant frequencies. (This would mean that the aerial might not be able to 'pull in' the digital signals with sufficient strength for the Freeview receiver to be able to 'lock on' to the signals).
When a new aerial is required for Freeview, it's usually recommended that the cabling should be replaced at the same time, as 'low-loss' coaxial cable will help to ensure that the signal is not degraded between the aerial and the Freeview receiver.
Chris