Daisychain

We've found a a prefab house on a beautiful plot of land that we'd like to buy. Our original plan was to apply for planning permission and get a self-build mortgage which would allow us to demolish it and build a "standard" house in its place. Unfortunately the vendor won't accept our offer on that basis so we're wondering if we can buy it on a tracker mortgage and then quite quickly switch to a self build mortgage once we own it? Any advice gratefully received!