You can fully transfer your existing ISA to NatWest mid way through a tax year if you advise NatWest this is what you are doing, they will arrange for the transfer to happen and your cash allowance of £3200 will come across to NatWest less the £400 you have already contributed this tax year. The statement you refer to just means you will not try and subscribe to 2 ISA's in one year thus doubling your allowance to £6400.