I've just done it. I was just under 2 years into a 4 year lease. I wanted to trade my 1.2tsi
Ibiza in for a 1.4tsi leon, due to the 1.2tsi being too slow and the
ibiza being too small.
I was £1500 in negative equity, however they wanted to get rid of the Leons due to the facelift coming out so they settled my existing finance and started a new agreement for the Leon, with a £3000 discount.
The only "fee" I had to pay was a £50 delivery fee as the car I wanted was in Doncaster and I'm in Leeds (it was a showroom car).
It was a very easy and painless process, however the dealer did say the only reason he has managed to get such a good deal for me was because my
Ibiza was in very good condition and black edition which apparently fetch a higher price and they tend to move them quickly.