never heard of it being done before, a mortgage has two principle elements to it,
property and mortgage holder, who must be the registered owner of the property, when a property is sold any mortgage secured on said property must be paid off from the proceeds of the sale.
The advantage of being able to do is if the mortgage is say 10 years into a 20 year term then the current repayments are reducing the capital by a higher amount than on a new mortgage which has little or no capital reduction in the first few years. Cant see a mortgage lender agreeing to it, as you will presumably be a new mortgage customer to them and should not benefit from interest already paid by previous mortgage holder.